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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Please, explain me this!</title>
<meta name="description" content="Javascript this: how to understand it">
<meta name="author" content="Gabriele D'Arrigo - gdarrigo@ebay.com">
<meta name="apple-mobile-web-app-capable" content="yes">
<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">
<link rel="stylesheet" href="css/reveal.css">
<link rel="stylesheet" href="css/theme/black.css" id="theme">
<!-- Theme used for syntax highlighting of code -->
<link rel="stylesheet" href="lib/css/zenburn.css">
<style>
pre.block-code {
box-shadow: none;
}
pre.block-code code {
padding: 10px;
background-color: rgba(63, 63, 63, 0.95);
}
pre.block-code code span.line-number {
margin-right: 20px;
}
.reveal .slides section .fragment.current-visible.current-fragment {
display: block;
}
.reveal .slides section .fragment.current-visible {
display: none;
}
section a.hidden-link {
color: #FFFFFF;
text-decoration: none;
}
a.hidden-link:active,
a.hidden-link:focus,
a.hidden-link:hover,
a.hidden-link:visited {
color: #FFFFFF;
cursor: text;
text-decoration: none;
}
.background-black {
background-color: rgba(0, 0, 0, 0.9);
padding: 25px;
}
</style>
</head>
<body>
<div class="reveal">
<div class="slides">
<section id="intro" data-background="http://i.giphy.com/l46CsaquyQudrz3Lq.gif">
<header>
<h1>This</h1>
<h3>The most infamous Javascript keyword</h3>
</header>
<pre class="block-code">
<code class="javascript line-numbers">
var self = this; // Dammi una lametta...
</code>
</pre>
</section>
<section id="context">
<h3>
This keyword can be used everywhere, not only in objects:
</h3>
<ul>
<li>
In functions
</li>
<li>
Outside functions (top-level scope)
</li>
<li>
In a string passed to eval() (not covered <a href="#/eval" class="hidden-link">here</a>)
</li>
</ul>
</section>
<section id="function">
<section id="functions-context">
<h3>
In functions
</h3>
<p>
The most common use are:
</p>
<ul>
<li>
Functions (<strong class="hljs-string">this</strong> refers to the global object, undefined in strict mode)
</li>
<li>
Constructors (<strong class="hljs-string">this</strong> refers to the object instance)
</li>
<li>
Methods (<strong class="hljs-string">this</strong> refers to the receiver of the method call)
</li>
</ul>
</section>
<section id="modes">
<h4>
Functions
</h4>
<p>
When you call a function the value of <strong class="hljs-string">this</strong> depends on the mode:
<strong class="hljs-keyword">normal</strong> or <strong class="hljs-keyword">strict</strong>
</p>
</section>
<section id="normal-mode">
In normal execution mode <strong class="hljs-string">this</strong> point to the global object: <br />
<strong class="hljs-keyword">window</strong> in browsers, <strong class="hljs-keyword">global</strong>
in Node.js or other server side environment.
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">function log() {
console.log(this);
}
log(); // window</code>
</pre>
</section>
<section id="strict-mode">
<p>
In strict mode the value of <strong class="hljs-string">this</strong> is <strong class="hljs-keyword">undefined</strong>
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
'use strict';
function log() {
console.log(this);
}
log(); // undefined</code>
</pre>
</section>
<section id="change-value-this">
<p>
We can arbitrarily change the value of <strong class="hljs-string">this</strong> with two function:
</p>
<ul>
<li>
<strong class="hljs-keyword">Function.prototype.call</strong>
</li>
<li>
<strong class="hljs-keyword">Function.prototype.apply</strong>
</li>
</ul>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
let robot = {
name: 'Robocop'
};
function kill(toKill) {
console.log(this.name + ' will kill ' + toKill);
}
kill.call(robot, 'you') // Robocop will kill you
kill.apply(robot, ['you']) // Robocop will kill you</code>
</pre>
</section>
<section id="constructor">
<h4>
Constructors
</h4>
<p>
A constructor is a function invoked with the <strong class="hljs-keyword">new</strong> operator that
return the instance of an object.
</p>
</section>
<section id="constructor-invocation">
<p>
When a constructor is invoked with <strong class="hljs-keyword">new</strong>,
<strong class="hljs-string">this</strong> is bound to the newly created object.
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
function Dog(name) {
this.name = name;
}
let pluto = new Dog('Pluto');
pluto.name; // 'Pluto'</code>
</pre>
</section>
<section id="methods">
<h4>
Methods
</h4>
<p>
<strong class="hljs-string">this</strong> refers to the receiver, the object on which the method has been called.
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
let obj = {
prop: 'a prop',
method() {
console.log(this.prop);
}
};
obj.method(); // 'a prop'</code>
</pre>
</section>
</section>
<section id="top-level">
<h3>
Top level scope
</h3>
<p>
In browsers, the top-level scope is the global scope and <strong class="hljs-string">this</strong> refers
to the global object (<strong class="hljs-keyword">window</strong>)
</p>
<pre class="block-code" contenteditable="true">
<code class="line-numbers">
<script>
console.log(this); // window
</script></code>
</pre>
</section>
<section id="mistakes-fixes">
<section id="common-mistakes">
<h3>
Common mistakes
</h3>
<ul>
<li>
Forgetting <strong class="hljs-keyword">new</strong> operator
</li>
<li>
Shadowing the value of <strong class="hljs-string">this</strong>
</li>
<li>
Extracting methods invocation
</li>
</ul>
</section>
<section id="forgetting-new">
<h4>
Forgetting <strong class="hljs-keyword">new</strong> operator
</h4>
<p>
When you call a constructor without the <strong class="hljs-keyword">new</strong> operator you are invoking
it like a real function! <br />
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
function Point(x, y) {
this.x = x;
this.y = y;
}
let point = Point(10, 20);
// Constructor doesn't return the object instance!
console.log(point); // undefined
// Global variables created! Gomblotto!1!1
window.x; // 10
window.y; // 20</code>
</pre>
</section>
<section id="forgetting-new-fix" data-background="http://i.giphy.com/90F8aUepslB84.gif">
<h4>
Fix
</h4>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
'use strict';
function Point(x, y) {
this.x = x;
this.y = y;
}
// TypeError: Cannot set property 'x' of undefined
let point = Point(10, 20);</code>
</pre>
</section>
<section id="shadowing">
<h4>
Shadowing the value of <strong class="hljs-string">this</strong>
</h4>
<p>
Sometimes we forgot about Javascript lexical scope!
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
let component = {
width: 800,
init() {
document.addEventListener('click', function(e) {
console.log(this.width);
});
}
};
// On document's click this.width is undefined
component.init();</code>
</pre>
</section>
<section id="shadowing-fix-one" data-background="http://i.giphy.com/ZFTKZ8zwj38gE.gif">
<h4>
FIX
</h4>
<p>
We can store the value of <strong class="hljs-string">this</strong>
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
let component = {
width: 800,
init() {
// Woooooo
let self = this;
document.addEventListener('click', function(e) {
console.log(self.width);
});
}
};
component.init();</code>
</pre>
</section>
<section id="shadowing-fix-two" data-background="http://i.giphy.com/rIq6ASPIqo2k0.gif">
<h4>
FIX
</h4>
<p>
We can use a sexy <strong class="hljs-keyword">arrow function</strong>*
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
let component = {
width: 800,
init() {
document.addEventListener('click', e => {
console.log(this.width);
});
}
};
component.init();</code>
</pre>
<small>*only for ES6 brave guys!</small>
</section>
<section id="shadowing-fix-three" data-background="http://i.giphy.com/Czak1L6uy06iI.gif">
<h4>
FIX
</h4>
<p>
Or use <strong class="hljs-keyword">Function.prototype.bind</strong>
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
let component = {
width: 800,
init() {
document.addEventListener('click', function(e) {
console.log(this.width);
}.bind(this));
}
};
component.init();</code>
</pre>
</section>
<section id="extracting">
<h4>
Extracting methods invocation
</h4>
<p>
If we retrieve the value of a method, instead of invoking it, we turn that method into a function <br />
pointing <strong class="hljs-string">this</strong> to the global object!
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
let counter = {
count: 0,
increment() {
this.count++;
console.log(this.count);
}
};
setInterval(counter.increment, 1000);</code>
</pre>
</section>
<section id="extracting-fix">
<h4>
Fix
</h4>
<p>
Use <strong class="hljs-keyword">Function.prototype.bind</strong> to create a new function with
<strong class="hljs-string">this</strong> bound
to the right object.
</p>
<pre class="block-code" contenteditable="true">
<code class="javascript line-numbers">
let counter = {
count: 0,
increment() {
this.count++;
console.log(this.count);
}
};
setInterval(counter.increment.bind(counter), 1000);</code>
</pre>
</section>
</section>
<section id="the-end" data-background="http://i.giphy.com/nguAwtOo4nxAY.gif">
<h2>
Thank you!
</h2>
<small>
<a class="hidden-link" href="#exercises">
Exercises
</a>
</small>
</section>
<section id="eval">
<section id="eval-context" data-background="http://i.giphy.com/oKVs1VY0MKfvO.gif">
<div class="background-black">
<h3>
The hidden slide on eval!
</h3>
<h4>
<strong class="hljs-keyword">eval</strong> function can be called:
</h4>
<ul>
<li>
Directly
</li>
<li>
Indirectly
</li>
</ul>
</div>
</section>
<section id="eval-directly">
<p>
When eval is called directly <strong class="hljs-string">this</strong> point to surrounding eval context
</p>
<pre class="block-code" contenteditable="true">
<code class="line-numbers">
let x = 'global';
function directly() {
let x = 'local';
console.log(eval('x')); // local
}
directly();
// Point depending on the context;
function nonStrict() {
console.log(eval('this')); // window
}
nonStrict();
function strict() {
'use strict';
console.log(eval('this')); // undefined
}
strict();</code>
</pre>
</section>
<section id="eval-indirectly">
<p>
When eval is called indirectly <strong class="hljs-string">this</strong> always point to the global object
</p>
<pre class="block-code" contenteditable="true">
<code class="line-numbers">
let x = 'global';
function indirectly() {
let x = 'locals';
console.log(window.eval('x'));
}
indirectly();
// Let's try with this
let obj = {
indirectly: function() {
console.log(window.eval('this'));
}
};
obj.indirectly();</code>
</pre>
</section>
</section>
<section id="exercises" data-background="http://i.giphy.com/cTdsso3i4oGdO.gif">
<div class="background-black">
<h2>Exercises</h2>
<ul>
<li>
<a href="http://codepen.io/anon/pen/bByGXr?editors=0012">
First
</a>
</li>
<li>
<a href="http://codepen.io/anon/pen/NbVWQa?editors=0012">
Second
</a>
</li>
<li>
<a href="http://codepen.io/anon/pen/rWgNpv?editors=0012">
oledata
</a>
</li>
</ul>
</div>
</section>
</div>
</div>
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