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JavaScript. W3Schools in English. Lessons for beginners

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ECMAScript 2017

The JavaScript naming convention started with ES1, ES2, ES3, ES5 and ES6.

But, ECMAScript 2016 and ECMAScript 2017 was not called ES7 and ES8.

Since 2016 new versions are named by year (ECMAScript 2016 / 2017 / 2018).

New Features in ECMAScript 2017

This chapter introduces the new features in ECMAScript 2017:


JavaScript String Padding

ECMAScript 2017 added two String methods: padStart and padEnd to support padding at the beginning and at the end of a string.

Example

let str = "5";
str = str.padStart(4,0);
// result is 0005
Try it Yourself »

Example

let str = "5";
str = str.padEnd(4,0);
// result is 5000
Try it Yourself »

String Padding is not supported in Internet Explorer.

Firefox and Safari were the first browsers with support for JavaScript string padding:

Chrome 57 Edge 15 Firefox 48 Safari 10 Opera 44
Mar 2017 Apr 2017 Aug 2016 Sep 2016 Mar 2017

JavaScript Object Entries

ECMAScript 2017 adds a new Object.entries method to objects.

The Object.entries() method returns an array of the key/value pairs in an object:

Example

const person = {
  firstName : "John",
  lastName : "Doe",
  age : 50,
  eyeColor : "blue"
};
document.getElementById("demo").innerHTML = Object.entries(person);
Try it Yourself »

Object.entries() makes it simple to use objects in loops:

Example

const fruits = {Bananas:300, Oranges:200, Apples:500};

let text = "";
for (let [fruit, value] of Object.entries(fruits)) {
text += fruit + ": " + value + "
";
}
Try it Yourself »

Object.entries() also makes it simple to convert objects to maps:

Example

const fruits = {Bananas:300, Oranges:200, Apples:500};

const myMap = new Map(Object.entries(fruits));
Try it Yourself »

Chrome and Firefox were the first browsers with support for Object.entries:

Chrome 47 Edge 14 Firefox 47 Safari 10.1 Opera 41
Jun 2016 Aug 2016 Jun 2016 Mar 2017 Oct 2016

JavaScript Object Values

Object.values are similar to Object.entries, but returns a single dimension array of the object values:

Example

const person = {
  firstName : "John",
  lastName : "Doe",
  age : 50,
  eyeColor : "blue"
};
document.getElementById("demo").innerHTML = Object.values(person);
Try it Yourself »

Firefox and Chrome were the first browsers with support for Object.values:

Chrome 54 Edge 14 Firefox 47 Safari 10.1 Opera 41
Oct 2016 Aug 2016 Jun 2016 Mar 2017 Oct 2016

JavaScript Async Functions

Waiting for a Timeout

async function myDisplay() {
  let myPromise = new Promise(function(myResolve, myReject) {
    setTimeout(function() { myResolve("I love You !!"); }, 3000);
  });
  document.getElementById("demo").innerHTML = await myPromise;
}

myDisplay();
Try it Yourself »

Firefox and Chrome were the first browsers with support for async JavaScript functions:

Chrome 55 Edge 15 Firefox 52 Safari 11 Opera 42
Dec 2016 Apr 2017 Mar 2017 Sep 2017 Dec 2016