Add ES2019 code samples

Author: sudheerj

README.md

@@ -1466,12 +1466,12 @@ Each proposal for an ECMAScript feature goes through the following maturity stag
 
    Whereas, **flatMap()** method combines `map()` and `flat()` into one method.  It first creates a new array with the return value of a given function and then concatenates all sub-array elements of the array.
    ```js
-   const numberArray = [[1], [2], [3], [4], [5]];
+   const numberArray1 = [[1], [2], [3], [4], [5]];
 
-   console.log(numberArray.flatMap(value => [value * 10]));  // [10, 20, 30, 40, 50]
+   console.log(numberArray1.flatMap(value => [value * 10]));  // [10, 20, 30, 40, 50]
    ```
 
-2. ### Object formEntries
+2. ### Object fromEntries
 
    In JavaScript, it is very commonn to transforming data from one format. ES2017 introduced `Object.entries()` method to objects into arrays.
 
@@ -1541,7 +1541,6 @@ Each proposal for an ECMAScript feature goes through the following maturity stag
  console.log(Symbol().description); // unefined
 
  console.log(Symbol.iterator.description); // "Symbol.iterator"
-
  ```
 
 5. ### Optional catch binding

es2015/13.set.js

@@ -0,0 +1,10 @@
+let mySet = new Set()
+
+mySet.add(1);
+mySet.add(2);
+mySet.add(2);
+mySet.add('some text here');
+mySet.add({one: 1, two: 2 , three: 3});
+console.log(mySet); // Set [ 1, 2, 'some text here', {one: 1, two: 2 , three: 3} ]
+console.log(mySet.size) // 4
+console.log(mySet.has(2)); // true

es2015/14.weakset.js

@@ -0,0 +1,18 @@
+let myUserSet = new WeakSet();
+
+let john = { name: "John" };
+let rocky = { name: "Rocky" };
+let alex = { name: "Alex" };
+let nick = { name: "Nick" };
+
+myUserSet.add(john);
+myUserSet.add(rocky);
+myUserSet.add(john);
+myUserSet.add(nick);
+
+console.log(myUserSet.has(john)); // true
+console.log(myUserSet.has(alex)); // false
+console.log(myUserSet.delete(nick));
+console.log(myUserSet.has(nick)); // false
+
+john = null;

es2015/15.map.js

@@ -0,0 +1,25 @@
+let typeMap = new Map();
+
+var keyObj = {'one': 1}
+
+typeMap.set('10', 'string');   // a string key
+typeMap.set(10, 'number');     // a numeric key
+typeMap.set(true, 'boolean'); // a boolean key
+typeMap.set(keyObj, 'object'); // an object key
+
+
+console.log(typeMap.get(10)   ); // number
+console.log(typeMap.get('10') ); // string
+console.log(typeMap.get(keyObj)) // object
+console.log(typeMap.get({'one': 1})) // undefined
+
+console.log(typeMap.size ); // 3
+
+for(let item of typeMap) {
+    console.log(item);
+}
+
+
+for(let item in typeMap) {
+    console.log(item);
+}

es2015/16.weakmap.js

@@ -0,0 +1,17 @@
+var weakMap = new WeakMap();
+
+var obj1  = {}
+var obj2  = {}
+
+
+weakMap.set(obj1, 1);
+weakMap.set(obj2, 2);
+weakMap.set({}, {"four": 4});
+
+console.log(weakMap.get(obj2)); // 2
+console.log(weakMap.has({})); // return false even though empty object exists as key. Because the keys have different references
+
+delete obj2;
+console.log(weakMap.get(obj2)); // 2
+weakMap.delete(obj1)
+console.log(weakMap.get(obj1)); //undefined

es2019/1.array-flat-flatmap.js

@@ -0,0 +1,15 @@
+// flat
+const numberArray = [[1, 2], [[3], 4], [5, 6]];
+const charArray = ['a', , 'b', , , ['c', 'd'], 'e'];
+const flattenedArrOneLevel = numberArray.flat(1);
+const flattenedArrTwoLevel = numberArray.flat(2);
+const flattenedCharArrOneLevel = charArray.flat(1);
+
+console.log(flattenedArrOneLevel);    // [1, 2, [3], 4, 5, 6]
+console.log(flattenedArrTwoLevel);    // [1, 2, 3, 4, 5, 6]
+console.log(flattenedCharArrOneLevel);    // ['a', 'b', 'c', 'd', 'e']
+
+// flatMap
+const numberArray1 = [[1], [2], [3], [4], [5]];
+
+console.log(numberArray1.flatMap(value => [value * 10]));  // [10, 20, 30, 40, 50]

es2019/2.object-fromentries.js

@@ -0,0 +1,10 @@
+// Iterable to objects
+const arr = [ ['a', '1'], ['b', '2'], ['c', '3'] ];
+const obj = Object.fromEntries(arr);
+console.log(obj); // { a: "1", b: "2", c: "3" }
+
+// URLParams
+const paramsString = 'param1=foo&param2=baz';
+const searchParams = new URLSearchParams(paramsString);
+
+Object.fromEntries(searchParams);    // => {param1: "foo", param2: "baz"}

es2019/3.trimstart-trimend.js

@@ -0,0 +1,3 @@
+let messageTwo = "   Hello World!!    ";
+console.log(messageTwo.trimStart()); //Hello World!!
+console.log(messageTwo.trimEnd()); //   Hello World!!

es2019/4.symbol-description.js

@@ -0,0 +1,9 @@
+console.log(Symbol('one').description); // one
+
+console.log(Symbol.for('one').description); // "one"
+
+console.log(Symbol('').description); // ''
+
+console.log(Symbol().description); // unefined
+
+console.log(Symbol.iterator.description); // "Symbol.iterator"

es2019/5.optional-catch-binding.js

@@ -0,0 +1,6 @@
+let isTheFeatureImplemented = false;
+try {
+    if(isFeatureSupported()) {
+        isTheFeatureImplemented = true;
+    }
+} catch (unused) {}

es2019/6.json-improvements.js

@@ -0,0 +1,5 @@
+// JSON Superset
+console.log(JSON.parse('"\u2028"')); // ''
+
+// Well Formed JSON.Stringify
+console.log(JSON.stringify("\uD800")); // '"\ud800"'

es2019/7.array-stable-sort.js

@@ -0,0 +1,14 @@
+const users = [
+    { name: "Albert",  age: 30 },
+    { name: "Bravo",   age: 30 },
+    { name: "Colin",   age: 30 },
+    { name: "Rock",    age: 50 },
+    { name: "Sunny",   age: 50 },
+    { name: "Talor",    age: 50 },
+    { name: "John",   age: 25 },
+    { name: "Kindo",  age: 25 },
+    { name: "Lary",   age: 25 },
+    { name: "Minjie",   age: 25 },
+    { name: "Nova",    age: 25 }
+]
+users.sort((a, b) => a.age - b.age);

es2019/8.function-tostring.js

@@ -0,0 +1,12 @@
+function sayHello(message) {
+    let msg = message;
+    //Print message
+    console.log(`Hello, ${msg}`);
+}
+
+console.log(sayHello.toString());
+// function sayHello(message) {
+//       let msg = message;
+//       //Print message
+//       console.log(`Hello, ${msg}`);
+//   }