Modern JavaScript Cheatsheet

_{Image Credits: Ahmad Awais ⚡️}

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Introduction

What is Javascript? How does it behave behind the scenes?

What is a JS Engine?

Interpretation vs Compilation

JS is a just in time compiled language

Runtime is bigger than just the engine

JS Runtime without browser

Execution Context

The call stack

Motivation

This document is a cheatsheet for JavaScript you will frequently encounter in modern projects and most contemporary sample code.

This guide is not intended to teach you JavaScript from the ground up, but to help developers with basic knowledge who may struggle to get familiar with modern codebases (or let's say to learn React for instance) because of the JavaScript concepts used.

Besides, I will sometimes provide personal tips that may be debatable but will take care to mention that it's a personal recommendation when I do so.

Note: Most of the concepts introduced here are coming from a JavaScript language update (ES2015, often called ES6). You can find new features added by this update here; it's very well done.

Complementary Resources

When you struggle to understand a notion, I suggest you look for answers on the following resources:

MDN (Mozilla Developer Network)
You don't know JS (book)
Eloquent JavaScript (book)
Douglas Crockford's blog
ES6 Features with examples
Wes Bos blog (ES6)
Javascript Basics for Beginners - a free Udacity course
Reddit (JavaScript)
Google to find specific blog and resources
StackOverflow

Notions

Variable declaration: var, const, let

In JavaScript, there are three keywords available to declare a variable, and each has its differences. Those are var, let and const.

Short explanation

Variables declared with const keyword can't be reassigned, while let and var can.

I recommend always declaring your variables with const by default, but with let if it is a variable that you need to mutate or reassign later.

	`</th> <th>Reassignable</th> <th>Mutable</th>`	Temporal Dead Zone
const	Block	No	Yes	Yes
let	Block	Yes	Yes	Yes
var	Function or Global	Yes	Yes	No

Scopes can be Block || Function || Global

Sample code

const person = "Nick";
person = "John" // Will raise an error, person can't be reassigned

let person = "Nick";
person = "John";
console.log(person) // "John", reassignment is allowed with let

Detailed explanation

The scope of a variable roughly means "where is this variable available in the code".

var

var declared variables are function scoped, meaning that when a variable is created in a function, everything in that function can access that variable. Besides, a function scoped variable created in a function can't be accessed outside this function.

I recommend you to picture it as if an X scoped variable meant that this variable was a property of X.

function myFunction() {
  var myVar = "Nick";
  console.log(myVar); // "Nick" - myVar is accessible inside the function
}
console.log(myVar); // Throws a ReferenceError, myVar is not accessible outside the function.

Still focusing on the variable scope, here is a more subtle example:

function myFunction() {
  var myVar = "Nick";
  if (true) {
    var myVar = "John";
    console.log(myVar); // "John"
    // actually, myVar being function scoped, we just erased the previous myVar value "Nick" for "John"
  }
  console.log(myVar); // "John" - see how the instructions in the if block affected this value
}
console.log(myVar); // Throws a ReferenceError, myVar is not accessible outside the function.

Besides, var declared variables are moved to the top of the scope at execution. This is what we call var hoisting.

This portion of code:

console.log(myVar) // undefined -- no error raised
var myVar = 2;

is understood at execution like:

var myVar;
console.log(myVar) // undefined -- no error raised
myVar = 2;

let

var and let are about the same, but let declared variables

are block scoped
are not accessible before they are assigned
can't be re-declared in the same scope

Let's see the impact of block-scoping taking our previous example:

function myFunction() {
  let myVar = "Nick";
  if (true) {
    let myVar = "John";
    console.log(myVar); // "John"
    // actually, myVar being block scoped, we just created a new variable myVar.
    // this variable is not accessible outside this block and totally independent
    // from the first myVar created !
  }
  console.log(myVar); // "Nick", see how the instructions in the if block DID NOT affect this value
}
console.log(myVar); // Throws a ReferenceError, myVar is not accessible outside the function.

Now, what it means for let (and const) variables for not being accessible before being assigned:

console.log(myVar) // raises a ReferenceError !
let myVar = 2;

By contrast with var variables, if you try to read or write on a let or const variable before they are assigned an error will be raised. This phenomenon is often called Temporal dead zone or TDZ.

Note: Technically, let and const variables declarations are being hoisted too, but not their assignation. Since they're made so that they can't be used before assignation, it intuitively feels like there is no hoisting, but there is. Find out more on this very detailed explanation here if you want to know more.

In addition, you can't re-declare a let variable:

let myVar = 2;
let myVar = 3; // Raises a SyntaxError

const

const declared variables behave like let variables, but also they can't be reassigned.

To sum it up, const variables:

are block scoped
are not accessible before being assigned
can't be re-declared in the same scope
can't be reassigned

const myVar = "Nick";
myVar = "John" // raises an error, reassignment is not allowed

const myVar = "Nick";
const myVar = "John" // raises an error, re-declaration is not allowed

But there is a subtlety : const variables are not immutable ! Concretely, it means that object and array const declared variables can be mutated.

For objects:

const person = {
  name: 'Nick'
};
person.name = 'John' // this will work ! person variable is not completely reassigned, but mutated
console.log(person.name) // "John"
person = "Sandra" // raises an error, because reassignment is not allowed with const declared variables

For arrays:

const person = [];
person.push('John'); // this will work ! person variable is not completely reassigned, but mutated
console.log(person[0]) // "John"
person = ["Nick"] // raises an error, because reassignment is not allowed with const declared variables

External resource

Strict Mode

Strict mode helps us write safe code. It can throw explicit errors when they happen and not silently go wrong

Without strict mode with wrong variable name

Note that non strict mode directly accepts the wrong variable name

With strict mode

Note that non strict mode directly accepts the not defined variable name

Another example

Note: linting can also help catch this: ESLint

Functions

Old school functions

Function declaration

function add (a, b){
 return a + b;
}

const result = add(1,2);
// result is set to 3

Function expressions

const add = function (a,b) {
 return a + b;
}

Here function evaluates to something and therefore its an expression. In JS functions are not primitive types. They just spit out values and so they can be assigned

Weird Note: Declared functions can be called before being defined (They're hoisted then) while Functions defined as expressions can't be defined.

Expressions lead to safer code

Arrow function

The ES6 JavaScript update has introduced arrow functions, which is another way to declare and use functions. This is closer to function expressions than function declarations.

Here are the benefits they bring:

More concise
this is picked up from surroundings
implicit return

Sample code

Concision and implicit return

function double(x) { return x * 2; } // Traditional way
console.log(double(2)) // 4

const double = x => x * 2; // Same function written as an arrow function with implicit return
console.log(double(2)) // 4

this reference

In an arrow function, this is equal to the this value of the enclosing execution context. Basically, with arrow functions, you don't have to do the "that = this" trick before calling a function inside a function anymore.

function myFunc() {
  this.myVar = 0;
  setTimeout(() => {
    this.myVar++;
    console.log(this.myVar) // 1
  }, 0);
}

Detailed explanation

Concision

Arrow functions are more concise than traditional functions in many ways. Let's review all the possible cases:

Implicit VS Explicit return

An explicit return is a function where the return keyword is used in its body.

  function double(x) {
    return x * 2; // this function explicitly returns x * 2, *return* keyword is used
  }

In the traditional way of writing functions, the return was always explicit. But with arrow functions, you can do implicit return which means that you don't need to use the keyword return to return a value.

  const double = (x) => {
    return x * 2; // Explicit return here
  }

Since this function only returns something (no instructions before the return keyword) we can do an implicit return.

  const double = (x) => x * 2; // Correct, returns x*2

To do so, we only need to remove the brackets and the return keyword. That's why it's called an implicit return, the return keyword is not there, but this function will indeed return x * 2.

Note: If your function does not return a value (with side effects), it doesn't do an explicit nor an implicit return.

Besides, if you want to implicitly return an object you must have parentheses around it since it will conflict with the block braces:

const getPerson = () => ({ name: "Nick", age: 24 })
console.log(getPerson()) // { name: "Nick", age: 24 } -- object implicitly returned by arrow function

Only one argument

If your function only takes one parameter, you can omit the parentheses around it. If we take back the above double code:

  const double = (x) => x * 2; // this arrow function only takes one parameter

Parentheses around the parameter can be avoided:

  const double = x => x * 2; // this arrow function only takes one parameter

No arguments

When there is no argument provided to an arrow function, you need to provide parentheses, or it won't be valid syntax.

  () => { // parentheses are provided, everything is fine
    const x = 2;
    return x;
  }

  => { // No parentheses, this won't work!
    const x = 2;
    return x;
  }

this reference

To understand this subtlety introduced with arrow functions, you must know how this behaves in JavaScript.

this function behavior

self = this pattern

ES6 way: Use the arrow function. This works because arrow function this is inherited from parent scope

In an arrow function, this is equal to the this value of the enclosing execution context. What it means is that an arrow function doesn't create a new this, it grabs it from its surrounding instead.

Without arrow function, if you wanted to access a variable from this in a function inside a function, you had to use the that = this or self = this trick.

For instance, using setTimeout function inside myFunc:

function myFunc() {
  this.myVar = 0;
  var that = this; // that = this trick
  setTimeout(
    function() { // A new *this* is created in this function scope
      that.myVar++;
      console.log(that.myVar) // 1

      console.log(this.myVar) // undefined -- see function declaration above
    },
    0
  );
}

But with arrow function, this is taken from its surrounding:

function myFunc() {
  this.myVar = 0;
  setTimeout(
    () => { // this taken from surrounding, meaning myFunc here
      this.myVar++;
      console.log(this.myVar) // 1
    },
    0
  );
}

Useful resources

Function default parameter value

Starting from ES2015 JavaScript update, you can set default value to your function parameters using the following syntax:

function myFunc(x = 10) {
  return x;
}
console.log(myFunc()) // 10 -- no value is provided so x default value 10 is assigned to x in myFunc
console.log(myFunc(5)) // 5 -- a value is provided so x is equal to 5 in myFunc

console.log(myFunc(undefined)) // 10 -- undefined value is provided so default value is assigned to x
console.log(myFunc(null)) // null -- a value (null) is provided, see below for more details

The default parameter is applied in two and only two situations:

No parameter provided
undefined parameter provided

In other words, if you pass in null the default parameter won't be applied.

Note: Default value assignment can be used with destructured parameters as well (see next notion to see an example)

External resource

Primitive vs Objects

Arrays

Two ways of intializing

Immutable and multi value

Only primitive values are immutable

The concept of immutability with const only applies to primitive values. Arrays are not primitive. This is related to how javascript stores variables in memory (to be explored later). 

So what is the use of const for an array?

The entire array itself can't be replaced however. That will still throw the const error. So we can use it in that context.

Immutability for python is also similar. Primitive values are immutable, whereas arrays and dictionaries are not.

Excerpt from https://realpython.com/python-pass-by-reference/#best-practice-use-dictionaries-and-lists

Lists and sets are mutable, as are dictionaries and other mapping types. Strings and tuples are not mutable. Attempting to modify an element of an immutable object will raise a `TypeError`.

Multi valued

Operations on arrays

Destructuring objects and arrays

Destructuring is a convenient way of creating new variables by extracting some values from data stored in objects or arrays.

To name a few use cases, destructuring can be used to destructure function parameters or this.props in React projects for instance.

Explanation with sample code

Object

Let's consider the following object for all the samples:

const person = {
  firstName: "Nick",
  lastName: "Anderson",
  age: 35,
  sex: "M"
}

Without destructuring

const first = person.firstName;
const age = person.age;
const city = person.city || "Paris";

With destructuring, all in one line:

const { firstName: first, age, city = "Paris" } = person; // That's it !

console.log(age) // 35 -- A new variable age is created and is equal to person.age
console.log(first) // "Nick" -- A new variable first is created and is equal to person.firstName
console.log(firstName) // ReferenceError -- person.firstName exists BUT the new variable created is named first
console.log(city) // "Paris" -- A new variable city is created and since person.city is undefined, city is equal to the default value provided "Paris".

Note : In const { age } = person;, the brackets after const keyword are not used to declare an object nor a block but is the destructuring syntax.

Function parameters

Destructuring is often used to destructure objects parameters in functions.

Without destructuring

function joinFirstLastName(person) {
  const firstName = person.firstName;
  const lastName = person.lastName;
  return firstName + '-' + lastName;
}

joinFirstLastName(person); // "Nick-Anderson"

In destructuring the object parameter person, we get a more concise function:

function joinFirstLastName({ firstName, lastName }) { // we create firstName and lastName variables by destructuring person parameter
  return firstName + '-' + lastName;
}

joinFirstLastName(person); // "Nick-Anderson"

Destructuring is even more pleasant to use with arrow functions:

const joinFirstLastName = ({ firstName, lastName }) => firstName + '-' + lastName;

joinFirstLastName(person); // "Nick-Anderson"

Array

Let's consider the following array:

const myArray = ["a", "b", "c"];

Without destructuring

const x = myArray[0];
const y = myArray[1];

With destructuring

const [x, y] = myArray; // That's it !

console.log(x) // "a"
console.log(y) // "b"

Destructuring as a trick to swap variable values

[a,b] = [b,a]

We can also skip during destructuring

const [a , , b] = [1,2,3];
// a = 1, b = 3

Syntax error if we destructure like this (starting with braces, JS assumes code block)

Solution: Wrap in parenthesis

Spr

Useful resources

Array methods - map / filter / reduce / find

Map, filter, reduce and find are array methods that are coming from a programming paradigm named functional programming.

To sum it up:

Array.prototype.map() takes an array, does something on its elements and returns an array with the transformed elements.
Array.prototype.filter() takes an array, decides element by element if it should keep it or not and returns an array with the kept elements only
Array.prototype.reduce() takes an array and aggregates the elements into a single value (which is returned)
Array.prototype.find() takes an array, and returns the first element that satisfies the provided condition.

I recommend to use them as much as possible in following the principles of functional programming because they are composable, concise and elegant.

With those four methods, you can avoid the use of for and forEach loops in most situations. When you are tempted to do a for loop, try to do it with map, filter, reduce and find composed. You might struggle to do it at first because it requires you to learn a new way of thinking, but once you've got it things get easier.

Sample code

const numbers = [0, 1, 2, 3, 4, 5, 6];
const doubledNumbers = numbers.map(n => n * 2); // [0, 2, 4, 6, 8, 10, 12]
const evenNumbers = numbers.filter(n => n % 2 === 0); // [0, 2, 4, 6]
const sum = numbers.reduce((prev, next) => prev + next, 0); // 21
const greaterThanFour = numbers.find((n) => n>4); // 5

Compute total grade sum for students with grades 10 or above by composing map, filter and reduce:

const students = [
  { name: "Nick", grade: 10 },
  { name: "John", grade: 15 },
  { name: "Julia", grade: 19 },
  { name: "Nathalie", grade: 9 },
];

const aboveTenSum = students
  .map(student => student.grade) // we map the students array to an array of their grades
  .filter(grade => grade >= 10) // we filter the grades array to keep those 10 or above
  .reduce((prev, next) => prev + next, 0); // we sum all the grades 10 or above one by one

console.log(aboveTenSum) // 44 -- 10 (Nick) + 15 (John) + 19 (Julia), Nathalie below 10 is ignored

Explanation

Let's consider the following array of numbers for our examples:

const numbers = [0, 1, 2, 3, 4, 5, 6];

Array.prototype.map()

const doubledNumbers = numbers.map(function(n) {
  return n * 2;
});
console.log(doubledNumbers); // [0, 2, 4, 6, 8, 10, 12]

What's happening here? We are using .map on the numbers array, the map is iterating on each element of the array and passes it to our function. The goal of the function is to produce and return a new value from the one passed so that map can replace it.

Let's extract this function to make it more clear, just for this once:

const doubleN = function(n) { return n * 2; };
const doubledNumbers = numbers.map(doubleN);
console.log(doubledNumbers); // [0, 2, 4, 6, 8, 10, 12]

Note : You will frequently encounter this method used in combination with arrow functions

const doubledNumbers = numbers.map(n => n * 2);
console.log(doubledNumbers); // [0, 2, 4, 6, 8, 10, 12]

numbers.map(doubleN) produces [doubleN(0), doubleN(1), doubleN(2), doubleN(3), doubleN(4), doubleN(5), doubleN(6)] which is equal to [0, 2, 4, 6, 8, 10, 12].

Note: If you do not need to return a new array and just want to do a loop that has side effects, you might just want to use a for / forEach loop instead of a map.

Array.prototype.filter()

const evenNumbers = numbers.filter(function(n) {
  return n % 2 === 0; // true if "n" is par, false if "n" isn't
});
console.log(evenNumbers); // [0, 2, 4, 6]

Note : You will frequently encounter this method used in combination with arrow functions

const evenNumbers = numbers.filter(n => n % 2 === 0);
console.log(evenNumbers); // [0, 2, 4, 6]

We are using .filter on the numbers array, filter is iterating on each element of the array and passes it to our function. The goal of the function is to return a boolean that will determine whether the current value will be kept or not. Filter then returns the array with only the kept values.

Array.prototype.reduce()

The reduce method goal is to reduce all elements of the array it iterates on into a single value. How it aggregates those elements is up to you.

const sum = numbers.reduce(
  function(acc, n) {
    return acc + n;
  },
  0 // accumulator variable value at first iteration step
);

console.log(sum) // 21

Note : You will frequently encounter this method used in combination with arrow functions

const sum = numbers.reduce((acc, n) => acc + n, 0);
console.log(sum) // 21

Just like for .map and .filter methods, .reduce is applied on an array and takes a function as the first parameter.

This time though, there are changes:

.reduce takes two parameters

The first parameter is a function that will be called at each iteration step.

The second parameter is the value of the accumulator variable (acc here) at the first iteration step (read next point to understand).

Function parameters

The function you pass as the first parameter of .reduce takes two parameters. The first one (acc here) is the accumulator variable, whereas the second parameter (n) is the current element.

The accumulator variable is equal to the return value of your function at the previous iteration step. At the first step of the iteration, acc is equal to the value you passed as .reduce second parameter.

At first iteration step

acc = 0 because we passed in 0 as the second parameter for reduce

n = 0 first element of the number array

Function returns acc + n --> 0 + 0 --> 0

At second iteration step

acc = 0 because it's the value the function returned at the previous iteration step

n = 1 second element of the number array

Function returns acc + n --> 0 + 1 --> 1

At third iteration step

acc = 1 because it's the value the function returned at the previous iteration step

n = 2 third element of the number array

Function returns acc + n --> 1 + 2 --> 3

At fourth iteration step

acc = 3 because it's the value the function returned at the previous iteration step

n = 3 fourth element of the number array

Function returns acc + n --> 3 + 3 --> 6

[...] At last iteration step

acc = 15 because it's the value the function returned at the previous iteration step

n = 6 last element of the number array

Function returns acc + n --> 15 + 6 --> 21

As it is the last iteration step, .reduce returns 21.

Array.prototype.find()

const greaterThanZero = numbers.find(function(n) {
  return n > 0; // return number just greater than 0 is present
});
console.log(greaterThanZero); // 1

Note : You will frequently encounter this method used in combination with arrow functions

We are using .find on the numbers array, .find is iterating on each element of the array and passes it to our function, until the condition is met. The goal of the function is to return the element that satisfies the current testing function. The .find method executes the callback function once for each index of the array until the callback returns a truthy value.

Note : It immediately returns the value of that element (that satisfies the condition) if found. Otherwise, returns undefined.

External Resource

Understanding map / filter / reduce in JS

Spread operator "..."

The spread operator ... has been introduced with ES2015 and is used to expand elements of an iterable (like an array) into places where multiple elements can fit.

Sample code

const arr1 = ["a", "b", "c"];
const arr2 = [...arr1, "d", "e", "f"]; // ["a", "b", "c", "d", "e", "f"]

function myFunc(x, y, ...params) {
  console.log(x);
  console.log(y);
  console.log(params)
}

myFunc("a", "b", "c", "d", "e", "f")
// "a"
// "b"
// ["c", "d", "e", "f"]

const { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }

const n = { x, y, ...z };
console.log(n); // { x: 1, y: 2, a: 3, b: 4 }

Explanation

In iterables (like arrays)

If we have the two following arrays:

const arr1 = ["a", "b", "c"];
const arr2 = [arr1, "d", "e", "f"]; // [["a", "b", "c"], "d", "e", "f"]

arr2 the first element is an array because arr1 is injected as is into arr2. But what we want is arr2 to be an array of letters. To do so, we can spread the elements of arr1 into arr2.

With spread operator

const arr1 = ["a", "b", "c"];
const arr2 = [...arr1, "d", "e", "f"]; // ["a", "b", "c", "d", "e", "f"]

Function rest parameter

In function parameters, we can use the rest operator to inject parameters into an array we can loop in. There is already an arguments object bound to every function that is equal to an array of all the parameters passed into the function.

function myFunc() {
  for (var i = 0; i < arguments.length; i++) {
    console.log(arguments[i]);
  }
}

myFunc("Nick", "Anderson", 10, 12, 6);
// "Nick"
// "Anderson"
// 10
// 12
// 6

But let's say that we want this function to create a new student with its grades and with its average grade. Wouldn't it be more convenient to extract the first two parameters into two separate variables, and then have all the grades in an array we can iterate over?

That's exactly what the rest operator allows us to do!

function createStudent(firstName, lastName, ...grades) {
  // firstName = "Nick"
  // lastName = "Anderson"
  // [10, 12, 6] -- "..." takes all other parameters passed and creates a "grades" array variable that contains them

  const avgGrade = grades.reduce((acc, curr) => acc + curr, 0) / grades.length; // computes average grade from grades

  return {
    firstName: firstName,
    lastName: lastName,
    grades: grades,
    avgGrade: avgGrade
  }
}

const student = createStudent("Nick", "Anderson", 10, 12, 6);
console.log(student);
// {
//   firstName: "Nick",
//   lastName: "Anderson",
//   grades: [10, 12, 6],
//   avgGrade: 9,33
// }

Note: createStudent function is bad because we don't check if grades.length exists or is different from 0. But it's easier to read this way, so I didn't handle this case.

Object properties spreading

For this one, I recommend you read previous explanations about the rest operator on iterables and function parameters.

const myObj = { x: 1, y: 2, a: 3, b: 4 };
const { x, y, ...z } = myObj; // object destructuring here
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }

// z is the rest of the object destructured: myObj object minus x and y properties destructured

const n = { x, y, ...z };
console.log(n); // { x: 1, y: 2, a: 3, b: 4 }

// Here z object properties are spread into n

External resources

Object property shorthand

When assigning a variable to an object property, if the variable name is equal to the property name, you can do the following:

const x = 10;
const myObj = { x };
console.log(myObj.x) // 10

Explanation

Usually (pre-ES2015) when you declare a new object literal and want to use variables as object properties values, you would write this kind of code:

const x = 10;
const y = 20;

const myObj = {
  x: x, // assigning x variable value to myObj.x
  y: y // assigning y variable value to myObj.y
};

console.log(myObj.x) // 10
console.log(myObj.y) // 20

As you can see, this is quite repetitive because the properties name of myObj are the same as the variable names you want to assign to those properties.

With ES2015, when the variable name is the same as the property name, you can do this shorthand:

const x = 10;
const y = 20;

const myObj = {
  x,
  y
};

console.log(myObj.x) // 10
console.log(myObj.y) // 20

External resources

Property shorthand - ES6 Features

Promises

A promise is an object which can be returned synchronously from an asynchronous function (ref).

Promises can be used to avoid callback hell, and they are more and more frequently encountered in modern JavaScript projects.

Sample code

const fetchingPosts = new Promise((res, rej) => {
  $.get("/posts")
    .done(posts => res(posts))
    .fail(err => rej(err));
});

fetchingPosts
  .then(posts => console.log(posts))
  .catch(err => console.log(err));

Explanation

When you do an Ajax request the response is not synchronous because you want a resource that takes some time to come. It even may never come if the resource you have requested is unavailable for some reason (404).

To handle that kind of situation, ES2015 has given us promises. Promises can have three different states:

Pending
Fulfilled
Rejected

Let's say we want to use promises to handle an Ajax request to fetch the resource X.

Create the promise

We firstly are going to create a promise. We will use the jQuery get method to do our Ajax request to X.

const xFetcherPromise = new Promise( // Create promise using "new" keyword and store it into a variable
  function(resolve, reject) { // Promise constructor takes a function parameter which has resolve and reject parameters itself
    $.get("X") // Launch the Ajax request
      .done(function(X) { // Once the request is done...
        resolve(X); // ... resolve the promise with the X value as parameter
      })
      .fail(function(error) { // If the request has failed...
        reject(error); // ... reject the promise with the error as parameter
      });
  }
)

As seen in the above sample, the Promise object takes an executor function which takes two parameters resolve and reject. Those parameters are functions which when called are going to move the promise pending state to respectively a fulfilled and rejected state.

The promise is in pending state after instance creation and its executor function is executed immediately. Once one of the function resolve or reject is called in the executor function, the promise will call its associated handlers.

Promise handlers usage

To get the promise result (or error), we must attach to it handlers by doing the following:

xFetcherPromise
  .then(function(X) {
    console.log(X);
  })
  .catch(function(err) {
    console.log(err)
  })

If the promise succeeds, resolve is executed and the function passed as .then parameter is executed.

If it fails, reject is executed and the function passed as .catch parameter is executed.

Note : If the promise has already been fulfilled or rejected when a corresponding handler is attached, the handler will be called, so there is no race condition between an asynchronous operation completing and its handlers being attached. (Ref: MDN)

External Resources

Template literals

Template literals is an expression interpolation for single and multiple-line strings.

In other words, it is a new string syntax in which you can conveniently use any JavaScript expressions (variables for instance).

Sample code

const name = "Nick";
`Hello ${name}, the following expression is equal to four : ${2+2}`;

// Hello Nick, the following expression is equal to four: 4

External resources

Tagged template literals

Template tags are functions that can be prefixed to a template literal. When a function is called this way, the first parameter is an array of the strings that appear between the template's interpolated variables, and the subsequent parameters are the interpolated values. Use a spread operator ... to capture all of them. (Ref: MDN).

Note : A famous library named styled-components heavily relies on this feature.

Below is a toy example on how they work.

function highlight(strings, ...values) {
  const interpolation = strings.reduce((prev, current) => {
    return prev + current + (values.length ? "<mark>" + values.shift() + "</mark>" : "");
  }, "");

  return interpolation;
}

const condiment = "jam";
const meal = "toast";

highlight`I like ${condiment} on ${meal}.`;
// "I like <mark>jam</mark> on <mark>toast</mark>."

A more interesting example:

function comma(strings, ...values) {
  return strings.reduce((prev, next) => {
    let value = values.shift() || [];
    value = value.join(", ");
    return prev + next + value;
  }, "");
}

const snacks = ['apples', 'bananas', 'cherries'];
comma`I like ${snacks} to snack on.`;
// "I like apples, bananas, cherries to snack on."

External resources

Imports / Exports

ES6 modules are used to access variables or functions in a module explicitly exported by the modules it imports.

I highly recommend to take a look at MDN resources on import/export (see external resources below), it is both straightforward and complete.

Explanation with sample code

Named exports

Named exports are used to export several values from a module.

Note : You can only name-export first-class citizens that have a name.

// mathConstants.js
export const pi = 3.14;
export const exp = 2.7;
export const alpha = 0.35;

// -------------

// myFile.js
import { pi, exp } from './mathConstants.js'; // Named import -- destructuring-like syntax
console.log(pi) // 3.14
console.log(exp) // 2.7

// -------------

// mySecondFile.js
import * as constants from './mathConstants.js'; // Inject all exported values into constants variable
console.log(constants.pi) // 3.14
console.log(constants.exp) // 2.7

While named imports looks like destructuring, they have a different syntax and are not the same. They don't support default values nor deep destructuring.

Besides, you can do aliases but the syntax is different from the one used in destructuring:

import { foo as bar } from 'myFile.js'; // foo is imported and injected into a new bar variable

Default import / export

Concerning the default export, there is only a single default export per module. A default export can be a function, a class, an object or anything else. This value is considered the "main" exported value since it will be the simplest to import. Ref: MDN

// coolNumber.js
const ultimateNumber = 42;
export default ultimateNumber;

// ------------

// myFile.js
import number from './coolNumber.js';
// Default export, independently from its name, is automatically injected into number variable;
console.log(number) // 42

Function exporting:

// sum.js
export default function sum(x, y) {
  return x + y;
}
// -------------

// myFile.js
import sum from './sum.js';
const result = sum(1, 2);
console.log(result) // 3

External resources

JavaScript this

this operator behaves differently than in other languages and is in most cases determined by how a function is called. (Ref: MDN).

This notion is having many subtleties and being quite hard, I highly suggest you to deep dive in the external resources below. Thus, I will provide what I personally have in mind to determine what this is equal to. I have learned this tip from this article written by Yehuda Katz.

function myFunc() {
  ...
}

// After each statement, you find the value of *this* in myFunc

myFunc.call("myString", "hello") // "myString" -- first .call parameter value is injected into *this*

// In non-strict-mode
myFunc("hello") // window -- myFunc() is syntax sugar for myFunc.call(window, "hello")

// In strict-mode
myFunc("hello") // undefined -- myFunc() is syntax sugar for myFunc.call(undefined, "hello")

var person = {
  myFunc: function() { ... }
}

person.myFunc.call(person, "test") // person Object -- first call parameter is injected into *this*
person.myFunc("test") // person Object -- person.myFunc() is syntax sugar for person.myFunc.call(person, "test")

var myBoundFunc = person.myFunc.bind("hello") // Creates a new function in which we inject "hello" in *this* value
person.myFunc("test") // person Object -- The bind method has no effect on the original method
myBoundFunc("test") // "hello" -- myBoundFunc is person.myFunc with "hello" bound to *this*

External resources

Class

JavaScript is a prototype-based language (whereas Java is class-based language, for instance). ES6 has introduced JavaScript classes which are meant to be a syntactic sugar for prototype-based inheritance and not a new class-based inheritance model (ref).

The word class is indeed error prone if you are familiar with classes in other languages. If you do, avoid assuming how JavaScript classes work on this basis and consider it an entirely different notion.

Since this document is not an attempt to teach you the language from the ground up, I will assume you know what prototypes are and how they behave. If you do not, see the external resources listed below the sample code.

Samples

Before ES6, prototype syntax:

var Person = function(name, age) {
  this.name = name;
  this.age = age;
}
Person.prototype.stringSentence = function() {
  return "Hello, my name is " + this.name + " and I'm " + this.age;
}

With ES6 class syntax:

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  stringSentence() {
    return `Hello, my name is ${this.name} and I am ${this.age}`;
  }
}

const myPerson = new Person("Manu", 23);
console.log(myPerson.age) // 23
console.log(myPerson.stringSentence()) // "Hello, my name is Manu and I'm 23

External resources

For prototype understanding:

For classes understanding:

`Extends` and `super` keywords

The extends keyword is used in class declarations or class expressions to create a class which is a child of another class (Ref: MDN). The subclass inherits all the properties of the superclass and additionally can add new properties or modify the inherited ones.

The super keyword is used to call functions on an object's parent, including its constructor.

super keyword must be used before the this keyword is used in constructor
Invoking super() calls the parent class constructor. If you want to pass some arguments in a class's constructor to its parent's constructor, you call it with super(arguments).
If the parent class have a method (even static) called X, you can use super.X() to call it in a child class.

Sample Code

class Polygon {
  constructor(height, width) {
    this.name = 'Polygon';
    this.height = height;
    this.width = width;
  }

  getHelloPhrase() {
    return `Hi, I am a ${this.name}`;
  }
}

class Square extends Polygon {
  constructor(length) {
    // Here, it calls the parent class' constructor with lengths
    // provided for the Polygon's width and height
    super(length, length);
    // Note: In derived classes, super() must be called before you
    // can use 'this'. Leaving this out will cause a reference error.
    this.name = 'Square';
    this.length = length;
  }

  getCustomHelloPhrase() {
    const polygonPhrase = super.getHelloPhrase(); // accessing parent method with super.X() syntax
    return `${polygonPhrase} with a length of ${this.length}`;
  }

  get area() {
    return this.height * this.width;
  }
}

const mySquare = new Square(10);
console.log(mySquare.area) // 100
console.log(mySquare.getHelloPhrase()) // 'Hi, I am a Square' -- Square inherits from Polygon and has access to its methods
console.log(mySquare.getCustomHelloPhrase()) // 'Hi, I am a Square with a length of 10'

Note : If we had tried to use this before calling super() in Square class, a ReferenceError would have been raised:

class Square extends Polygon {
  constructor(length) {
    this.height; // ReferenceError, super needs to be called first!

    // Here, it calls the parent class' constructor with lengths
    // provided for the Polygon's width and height
    super(length, length);

    // Note: In derived classes, super() must be called before you
    // can use 'this'. Leaving this out will cause a reference error.
    this.name = 'Square';
  }
}

External Resources

Async Await

In addition to Promises, there is a new syntax you might encounter to handle asynchronous code named async / await.

The purpose of async/await functions is to simplify the behavior of using promises synchronously and to perform some behavior on a group of Promises. Just as Promises are similar to structured callbacks, async/await is similar to combining generators and promises. Async functions always return a Promise. (Ref: MDN)

Note : You must understand what promises are and how they work before trying to understand async / await since they rely on it.

Note 2: await must be used in an async function, which means that you can't use await in the top level of our code since that is not inside an async function.

Sample code

async function getGithubUser(username) { // async keyword allows usage of await in the function and means function returns a promise
  const response = await fetch(`https://api.github.com/users/${username}`); // Execution is paused here until the Promise returned by fetch is resolved
  return response.json();
}

getGithubUser('mbeaudru')
  .then(user => console.log(user)) // logging user response - cannot use await syntax since this code isn't in async function
  .catch(err => console.log(err)); // if an error is thrown in our async function, we will catch it here

Explanation with sample code

Async / Await is built on promises but they allow a more imperative style of code.

The async operator marks a function as asynchronous and will always return a Promise. You can use the await operator in an async function to pause execution on that line until the returned Promise from the expression either resolves or rejects.

async function myFunc() {
  // we can use await operator because this function is async
  return "hello world";
}

myFunc().then(msg => console.log(msg)) // "hello world" -- myFunc's return value is turned into a promise because of async operator

When the return statement of an async function is reached, the Promise is fulfilled with the value returned. If an error is thrown inside an async function, the Promise state will turn to rejected. If no value is returned from an async function, a Promise is still returned and resolves with no value when execution of the async function is complete.

await operator is used to wait for a Promise to be fulfilled and can only be used inside an async function body. When encountered, the code execution is paused until the promise is fulfilled.

Note : fetch is a function that returns a Promise that allows to do an AJAX request

Let's see how we could fetch a github user with promises first:

function getGithubUser(username) {
  return fetch(`https://api.github.com/users/${username}`).then(response => response.json());
}

getGithubUser('mbeaudru')
  .then(user => console.log(user))
  .catch(err => console.log(err));

Here's the async / await equivalent:

async function getGithubUser(username) { // promise + await keyword usage allowed
  const response = await fetch(`https://api.github.com/users/${username}`); // Execution stops here until fetch promise is fulfilled
  return response.json();
}

getGithubUser('mbeaudru')
  .then(user => console.log(user))
  .catch(err => console.log(err));

async / await syntax is particularly convenient when you need to chain promises that are interdependent.

For instance, if you need to get a token in order to be able to fetch a blog post on a database and then the author informations:

Note : await expressions needs to be wrapped in parentheses to call its resolved value's methods and properties on the same line.

async function fetchPostById(postId) {
  const token = (await fetch('token_url')).json().token;
  const post = (await fetch(`/posts/${postId}?token=${token}`)).json();
  const author = (await fetch(`/users/${post.authorId}`)).json();

  post.author = author;
  return post;
}

fetchPostById('gzIrzeo64')
  .then(post => console.log(post))
  .catch(err => console.log(err));

Error handling

Unless we add try / catch blocks around await expressions, uncaught exceptions – regardless of whether they were thrown in the body of your async function or while it’s suspended during await – will reject the promise returned by the async function. Using the throw statement in an async function is the same as returning a Promise that rejects. (Ref: PonyFoo).

Note : Promises behave the same!

With promises, here is how you would handle the error chain:

function getUser() { // This promise will be rejected!
  return new Promise((res, rej) => rej("User not found !"));
}

function getAvatarByUsername(userId) {
  return getUser(userId).then(user => user.avatar);
}

function getUserAvatar(username) {
  return getAvatarByUsername(username).then(avatar => ({ username, avatar }));
}

getUserAvatar('mbeaudru')
  .then(res => console.log(res))
  .catch(err => console.log(err)); // "User not found !"

The equivalent with async / await:

async function getUser() { // The returned promise will be rejected!
  throw "User not found !";
}

async function getAvatarByUsername(userId) => {
  const user = await getUser(userId);
  return user.avatar;
}

async function getUserAvatar(username) {
  var avatar = await getAvatarByUsername(username);
  return { username, avatar };
}

getUserAvatar('mbeaudru')
  .then(res => console.log(res))
  .catch(err => console.log(err)); // "User not found !"

External resources

Truthy / Falsy

In JavaScript, a truthy or falsy value is a value that is being casted into a boolean when evaluated in a boolean context. An example of boolean context would be the evaluation of an if condition:

Every value will be casted to true unless they are equal to:

false
0
"" (empty string)
null
undefined
NaN

Here are examples of boolean context:

if condition evaluation

if (myVar) {}

myVar can be any first-class citizen (variable, function, boolean) but it will be casted into a boolean because it's evaluated in a boolean context.

After logical NOT ! operator

This operator returns false if its single operand can be converted to true; otherwise, returns true.

!0 // true -- 0 is falsy so it returns true
!!0 // false -- 0 is falsy so !0 returns true so !(!0) returns false
!!"" // false -- empty string is falsy so NOT (NOT false) equals false

With the Boolean object constructor

new Boolean(0) // false
new Boolean(1) // true

In a ternary evaluation

myVar ? "truthy" : "falsy"

myVar is evaluated in a boolean context.

Be careful when comparing 2 values. The object values (that should be cast to true) is not being casted to Boolean but it forced to convert into a primitive value one using ToPrimitives specification. Internally, when an object is compared to Boolean value like [] == true, it does [].toString() == true so...

let a = [] == true // a is false since [].toString() give "" back.
let b = [1] == true // b is true since [1].toString() give "1" back.
let c = [2] == true // c is false since [2].toString() give "2" back.

External resources

Anamorphisms and Catamorphisms

Anamorphisms

Anamorphisms are functions that map from some object to a more complex structure containing the type of the object. It is the process of unfolding a simple structure into a more complex one. Consider unfolding an integer to a list of integers. The integer is our initial object and the list of integers is the more complex structure.

Sample code

function downToOne(n) {
  const list = [];

  for (let i = n; i > 0; --i) {
    list.push(i);
  }

  return list;
}

downToOne(5)
  //=> [ 5, 4, 3, 2, 1 ]

Catamorphisms

Catamorphisms are the opposite of Anamorphisms, in that they take objects of more complex structure and fold them into simpler structures. Take the following example product which take a list of integers and returns a single integer.

Sample code

function product(list) {
  let product = 1;

  for (const n of list) {
    product = product * n;
  }

  return product;
}

product(downToOne(5)) // 120

External resources

Generators

Another way to write the downToOne function is to use a Generator. To instantiate a Generator object, one must use the function * declaration. Generators are functions that can be exited and later re-entered with its context (variable bindings) saved across re-entrances.

For example, the downToOne function above can be rewritten as:

function * downToOne(n) {
  for (let i = n; i > 0; --i) {
    yield i;
  }
}

[...downToOne(5)] // [ 5, 4, 3, 2, 1 ]

Generators return an iterable object. When the iterator's next() function is called, it is executed until the first yield expression, which specifies the value to be returned from the iterator or with yield*, which delegates to another generator function. When a return expression is called in the generator, it will mark the generator as done and pass back as the return value. Further calls to next() will not return any new values.

Sample code

// Yield Example
function * idMaker() {
  var index = 0;
  while (index < 2) {
    yield index;
    index = index + 1;
  }
}

var gen = idMaker();

gen.next().value; // 0
gen.next().value; // 1
gen.next().value; // undefined

The yield* expression enables a generator to call another generator function during iteration.

// Yield * Example
function * genB(i) {
  yield i + 1;
  yield i + 2;
  yield i + 3;
}

function * genA(i) {
  yield i;
  yield* genB(i);
  yield i + 10;
}

var gen = genA(10);

gen.next().value; // 10
gen.next().value; // 11
gen.next().value; // 12
gen.next().value; // 13
gen.next().value; // 20

// Generator Return Example
function* yieldAndReturn() {
  yield "Y";
  return "R";
  yield "unreachable";
}

var gen = yieldAndReturn()
gen.next(); // { value: "Y", done: false }
gen.next(); // { value: "R", done: true }
gen.next(); // { value: undefined, done: true }

External resources

Mozilla MDN Web Docs, Iterators and Generators

Static Methods

Short explanation

The static keyword is used in classes to declare static methods. Static methods are functions in a class that belongs to the class object and are not available to any instance of that class.

Sample code

class Repo {
  static getName() {
    return "Repo name is modern-js-cheatsheet"
  }
}

// Note that we did not have to create an instance of the Repo class
console.log(Repo.getName()) // Repo name is modern-js-cheatsheet

let r = new Repo();
console.log(r.getName()) // Uncaught TypeError: r.getName is not a function

Detailed explanation

Static methods can be called within another static method by using the this keyword, this doesn't work for non-static methods though. Non-static methods cannot directly access static methods using the this keyword.

Calling other static methods from a static method.

To call a static method from another static method, the this keyword can be used like so;

class Repo {
  static getName() {
    return "Repo name is modern-js-cheatsheet"
  }

  static modifyName() {
    return this.getName() + '-added-this'
  }
}

console.log(Repo.modifyName()) // Repo name is modern-js-cheatsheet-added-this

Calling static methods from non-static methods.

Non-static methods can call static methods in 2 ways;

Using the class name.

To get access to a static method from a non-static method we use the class name and call the static method like a property. e.g ClassName.StaticMethodName

class Repo {
  static getName() {
    return "Repo name is modern-js-cheatsheet"
  }

  useName() {
    return Repo.getName() + ' and it contains some really important stuff'
  }
}

// we need to instantiate the class to use non-static methods
let r = new Repo()
console.log(r.useName()) // Repo name is modern-js-cheatsheet and it contains some really important stuff

Using the constructor

Static methods can be called as properties on the constructor object.

class Repo {
  static getName() {
    return "Repo name is modern-js-cheatsheet"
  }

  useName() {
    // Calls the static method as a property of the constructor
    return this.constructor.getName() + ' and it contains some really important stuff'
  }
}

// we need to instantiate the class to use non-static methods
let r = new Repo()
console.log(r.useName()) // Repo name is modern-js-cheatsheet and it contains some really important stuff

External resources

Glossary

Scope

The context in which values and expressions are "visible," or can be referenced. If a variable or other expression is not "in the current scope," then it is unavailable for use.

Source: MDN

Variable mutation

A variable is said to have been mutated when its initial value has changed afterward.

var myArray = [];
myArray.push("firstEl") // myArray is being mutated

A variable is said to be immutable if it can't be mutated.

Check MDN Mutable article for more details.

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Modern JavaScript Cheatsheet

Introduction

What is Javascript? How does it behave behind the scenes?

What is a JS Engine?

Interpretation vs Compilation

JS is a just in time compiled language

Runtime is bigger than just the engine

JS Runtime without browser

Execution Context

The call stack

Motivation

Complementary Resources

Table of Contents

Notions

Variable declaration: var, const, let

Short explanation

Sample code

Detailed explanation

var

let

const

External resource

Strict Mode

Functions

Old school functions

Arrow function

Sample code

Detailed explanation

Concision

this reference

this function behavior

Useful resources

Function default parameter value

External resource

Primitive vs Objects

Arrays

Destructuring objects and arrays

Explanation with sample code

Useful resources

Array methods - map / filter / reduce / find

Sample code

Explanation

Array.prototype.map()

Array.prototype.filter()

Array.prototype.reduce()

At first iteration step

At second iteration step

At third iteration step

At fourth iteration step

[...] At last iteration step

Array.prototype.find()

External Resource

Spread operator "..."

Sample code

Explanation

In iterables (like arrays)

Function rest parameter

Object properties spreading

External resources

Object property shorthand

Explanation

External resources

Promises

Sample code

Explanation

Create the promise

Promise handlers usage

External Resources

Template literals

Sample code

External resources

Tagged template literals

External resources

Imports / Exports

`Extends` and `super` keywords

Packages