Learn the Top 10 best ECMAScript 2015 features

Lee Boonstra @ladysign

“What you know as JavaScript in browsers and Node.js is actually a superset of ECMAScript.

Browsers and Node.js add more functionality through additional objects and methods, but the core of the language remains as defined in ECMAScript."

Sit back, and relax

Terminology

•JavaScript - The language name

•ECMAScript - The name of the standard

•Transpiler - Transforms ES2015 code to the JavaScript that currently supported in our browsers.

•TypeScript - A programming language, which is a superset of JS that aims to align with ES2015, by Microsoft. Compiles to plain JavaScript, because of definition files it can contain type information.

History

About Compatibility

You can find an overview of compatibility, in one of these ECMAScript 2015 compatibility tables: http://kangax.github.io/compat-table/es6/

ES2015 Implementations:

• V8 (Chrome, Node.js, Opera)

• JavaScript Core (Webkit, Safari)

• Chakra (MS Edge)

• SpiderMonkey (Firefox)

• Mobile (iOS 9+, Android 4.4+)

Sencha

Top 10 ES2015 features

10. Block scoped constructs

9. Default parameters

8. Template Literals

7. Arrow functions

6. Destructuring Assignment

5. For of loop

4. New Methods

3. Promises

2. Classes

1. Modules

10. Block Scoped Constructs

What’s hoisting?

Variable declarations using var are treated as if they are at the top of the function (or global

scope, if declared outside of a function) regardless of where these were placed.

Hoisting explained

You might have seen something like this before:foo() executes here:

(function() {console.log(typeof foo); // prints

"function"foo(); // logs "hello

sencha"function foo() { console.log("hello sencha!");

}})();

Hoisting explained

But here it won't...

(function() {console.log(typeof foo);

// prints "undefined"

foo(); // Error: foo is not a function

var foo = function() { console.log("hello sencha!");

}})();

Hoisting explained

This is how the browser interprets it:

// prints "undefined"

(function() {var foo; console.log(typeof foo); function anon() {console.log("hello

sencha!");} //some anonymous

functionfoo();

// Error: foo is not a function

//bind anonymous function to foo

foo = anon;})();

Get the code: http://bit.ly/2aAvlpS

Block level declarations

ECMAScript 2015 defines block-level declarations.

With those you can declare variables that are inaccessible outside of a given block scope. Block scopes, also called lexical scopes, are created:

1. Inside of a function

2. Inside of a block (indicated by the { and } characters)

Let Declaration

The let declaration syntax is almost the same as the syntax for var.

(function() {console.log(typeof foo);

// ReferenceError: can't access

foo(); // I won't even get here.let foo = function()

{ console.log("hello sencha!");

}})();

Get the code: http://bit.ly/2aAvlpS

Can only be declared once in the same scope

Below gives a syntax error because you can’t re- declare an identifier within the same (block) scope:

var baz = "sencha";let baz = "extjs";

SyntaxError: redeclaration of non-configurable global property baz

Creates new identifier in a new block scope

This time it won't throw an error:

var baz = "sencha";

if(true){let baz = "extjs";//let baz is extjs, but var baz is still sencha

}console.log(baz); //logs sencha

extjs

Note, it doesn’t log , because

let baz

is only availablewithin its own block

scope.

Constant declarations

You can also define declarations in ES2015 with the const declaration syntax.

These values cannot be changed once set.

const LIVEURL = "http://www.sencha.com/feed.json";

Example of errors

The first example is missing initialization. The second example throws a type error because you can’t assign a new variable to a constant.

const LIVEURL= "http://www.sencha.com/feed.json";

LIVEURL = "http://www.leeboonstra.com/feed.json";//TypeError: invalid assignment to const `liveUrl'

const LOCALURL;//syntax error, missing = in const declaration

This will run perfectly fine

It prevents the modification of the binding, not the value itself.

const URLS = {};URLS.LIVE = "http://www.sencha.com/feed.json"; URLS.LIVE ="http://www.leeboonstra.com/feed.json"; URLS.LOCAL = "http://localhost/feed.json"; console.log(URLS);//Object { live: "http://www.leeboonstra.com/feed.json",//local: "http://localhost/feed.json" }

Gotcha

• var declarations are hoisted to the top of the function scope

• Use let for setting declarations in block scopes

• Use const for declarations that can’t change the binding

• Be careful when you are replacing var for let and const.• let and const can create dead zones

• Variables are meant for changing the values! A constant not.

9. Default Parameters

About Default Parameters

Prior to ECMAScript 2015, code would look like this script. You'll have to pass in the url parameter, but timeout and callback are optional.

function makeRequest(url, timeout, callback) {timeout = (typeof timeout !== "undefined") ? timeout : 2000; callback = callback || function() {};

//do something}

Default function parameters

With ECMAScript 2015, you can actually provide default function parameters:

function makeRequest (url, timeout = 2000, callback = function() {}) {

//do something}

Get the code: http://bit.ly/2axL0J5

Gotcha

• Default parameters, can keep your function bodies clean, without re-defining and assigning values to inner variables.

8. Template Literals

ES5 JavaScript Strings

•JavaScript always lacked, when we are talking about Strings, especially when talking about:

- Multiline strings

- HTML escaping

- Basic string formatting

Multiline Strings

Example ES5 way of dealing with multiline Strings:

var product = "Sencha";var myText = product + " provides the industry's most " + "comprehensive collection of high‐performance, " + "customizable UI widgets.These \"widgets\" include: " + "HTML5 grids, trees, lists, forms, menus, toolbars," + "panels, windows, and much more.";

Multiline Strings

Here's how it looks in ES2015:

let product = "Sencha";let myText = `${product} provides the industry's most comprehensive collection of high‐performance, customizable UI widgets. These "widgets" include HTML5 grids, trees, lists, forms, menus, toolbars, panels, windows, and much more.`;Get the code: http://bit.ly/2azCR78

Tagged Templates

Template functions, a template tag performs a transformation on the template literal and returns the final string value.

function mytag(arrLiterals){return arrLiterals[0].toUpperCase();

}

let message = mytag`hello world!`; //HELLO WORLD!

Get the code: http://bit.ly/2aT6eiO

Gotcha

• `Multiline Strings` FTW!• For easy mixing single and double quotes

• String substitution (placeholders in Strings)

• Tagged templates are like template functions in JS.

7. Arrow Functions

Arrow Functions: (x,y) => x + y

WHAT EVEN IS THIS?

Arrow Functions

Arrow functions, are a new syntax for functions, defined with an arrow.Consider the following ES5 functions:

var calculate = function(x,y){ return x + y;

}var sayHello = function(name){ return "Hello " + name;

}

calculate(5,10); //returns: 15sayHello('Lee'); //returns: Hello Lee

Arrow Functions

And compare these to the following new syntax:

let calculate = (x,y) => {return x + y;

}let sayHello = (name) => { return "Hello " + name;}

calculate(5,10); //returns: 15sayHello('Lee'); //returns: Hello Lee

Arrow Functions

This would work as well:

let calculate = (x,y) => x + y;let sayHello = (name) => "Hello " + name;

Get the code: http://bit.ly/2b7G8Gz

Be aware

Arrow functions could slightly behave different compared to traditional JavaScript functions:

• No this, super, arguments• No duplicated named arguments• Can not be called with new• No prototype• You can’t change this• Lexical this

Lexical

Remember var me = this; or var self = this; ? Often used when working with callbacks, to refer to the original scope, while the current scope changed, because of a callback.

this

function VideoGame(){ //ES5var me = this; me.title = "Uncharted", me.version = 3;

setInterval(function() {return me.title + " " + me.version++; console.log("1:", this); //Context is Window console.log("2:", me); //Context is VideoGame()

}, 5000);}

Lexical

• An arrow function does not create it's own this scope, rather it captures the this value of the enclosing scope.

function VideoGame(){ this.title = “Uncharted”;

this.version = 3;setInterval(() => {console.log(this.title + " " + this.version++);

// |this| properly refers to the VideoGame object

}, 5000);}

Get the code: http://bit.ly/2azNHae

this

Gotcha

• Arrow functions =>, write less code

• Ideal for callback function / inline functions.

• When working in teams, make sure everyone understands.

• Be aware:

• No arguments

• Can not create instance with new

• No prototype

• Lexical this

I’ve created examples for this, which I will leave in the slidedeck.

6. Destructuring Assignment

Consider this Objectvar myobject = { name: "Sencha", logo: { small: { png: 'http://path/to/small.png', jpg: 'http://path/to/small.jpg'

}, large: { png: 'http://path/to/large.png', jpg: 'http://path/to/large.jpg'

} }}

ES5: Working with Objects

In ES5 you would do something like this:

var logo = myobject.logo.small.png;var small = myobject.logo.small;var company = myobject.name;

console.log(logo); //Senchaconsole.log(small); // { png: "http://path/to/small.png",

//jpg: "http://path/to/small.jpg" }

console.log(company); //http://path/to/small.jpg

ES2015: Object Destructuring

Compare this to the ES2015 way, with object destructuring:

let { name, logo : { small : { jpg } } } = myobject;

console.log(name); //Senchaconsole.log(small); // { png: "http://path/to/small.png",

//jpg: "http://path/to/small.jpg" }console.log(jpg); //http://path/to/small.jpgGet the code: http://bit.ly/2aDgsmT

ES2015: Array Destructuring

You can also destruct arrays:

let myarray = ["Ext JS", "GXT", "Sencha Test"];let [myvar1, myvar2, myvar3] = myarray;

console.log(myvar1, myvar2, myvar3) //Ext JS GXT Sencha TestGet the code: http://bit.ly/2aA099W

Gotcha

• Extract data from Arrays and Objects• Ideal for when working with deeply nested Objects. No longer you

need to create extra variables with dot-notation.

5. for..of loop

ES5: For loops

ES5 has various ways to loop through

array elements and objects.

• ES5 for loop

• ES5 forEach() loop,

can’t break, continue and return• ES5 for in, iterates through object properties

var array = ["a", "b", "c"]; Var obj = {

firstname: "Lee", lastname: "Boonstra" };//1)for (var i = 0; i < array.length; i++) {     console.log(array[i]);}//2)array.forEach(function(element) {

console.log(element); });//3)for(prop in obj) {

console.log(prop + " = " + obj[prop]);}

For of loop

New in ES2015, the for...of statement creates a loop iterating over iterable objects (including Array, Map, Set, String, arguments object etc…)

var array = ["a","b","c","d"];for(i of array){ console.log(i);

if(i == "c") return;}

Gotcha

• ES6 For of loop, most concise, direct syntax yet for looping through array elements

• ..but, the old-fashioned ES5 for loop just performs better

• https://www.incredible-web.com/blog/performance-of-for-loops-with-javascript/

4. New methods

ES5: Check equality

console.log(5 == 5); // true console.log(5 == "5"); // true console.log(5 === 5); // true console.log(5 === "5"); // false

console.log(+0 == ‐0); // true <‐‐‐‐‐‐‐‐‐‐WAIT WHAT?!! console.log(+0 === ‐0); // true <‐‐‐‐‐‐‐‐‐WAIT WHAT?!! console.log(NaN == NaN); // false <‐‐‐‐‐‐‐‐WAIT WHAT?!!

ES2015: Object.is()

This method performs strict equality on any value, effectively becoming a safer version of === when dealing with special JavaScript values:

console.log(Object.is(5, 5)); // trueconsole.log(Object.is(5, "5")); // false

console.log(Object.is(+0, ‐0)); // false <‐‐‐‐ Oh, OK!console.log(Object.is(NaN, NaN)); // true <‐‐‐ Oh, OK!

Get the code: http://bit.ly/2b8o9AB

ES2015: Object.assign()

This method makes it easier to change multiple properties on a single object at once.

function Observable() { } Observable.prototype = {

constructor: EventTarget,on: function() { console.log("An event happened.") }

}

let SomeObject = {}Object.assign(SomeObject, Observable.prototype);

SomeObject.on("customevent"); //An event happened.

Get the code: http://bit.ly/2b2KB0E

More New Methods

There are much more new methods available in ES2015.

New methods on the following prototypes:

• Object

• Array

• Number

• Math

Gotcha

• Object.is() – Equality checking, the right way

• Object.asign() – Ideal for coding Mixin patterns

• More new methods on prototypes of:

• Object - http://bit.ly/2cbFU3l

• Array - http://bit.ly/2co0yNn

• Number & Math - http://bit.ly/2cnDRb0

3. Promises

Async JavaScript

Promises are new to ECMAScript 2015, and helps you with dealing with asynchronous code.

JavaScript code is single threaded:

• Code executes from top to bottom

• Right after each other

• 2 bits of code cannot run at the same time

Event Loop

• The Event Loop is a queue of callback functions. When the JS engine sees an async method, that method is pushed into the queue.

I HEARD YOU LIKE CALLBACKS

SO I PUT CALLBACKS IN YOUR CALLBACKS

SO YOU CAN CALL THEM BACK

ES5: Callbacks

//Node.js Examplevar done = function(err, jobs){ res.render("myview", {user : req.user, jobs: jobs

});};

getJobs = function(req, done){

// Something asynchronous: MongoDB,// request jobs from a databaseJob.findJobsOfUser(req.user._id, function(err, jobs){

if(err) throw err; done(null, jobs);

});}

var jobs = getJobs(req, done);

ES2015: Promises (1)

getJobs(req).then(function(jobs){if(jobs){

//now if I want to chain this with more calls//i just run the res.render finallyres.render("profile", { user: req.user, jobs: jobs

});}}, function(error)

{ throw error;});

ES2015: Promises (2)

function getJobs(req){var promise = new Promise(function (resolve, reject) {//Something asynchronous: MongoDB,//request jobs from a databaseJob.findJobsOfUser(req.user._id, function (err, jobs) {if (err) reject(err); //reject the promiseelse resolve(jobs); //or resolve the promise

});});return promise;

}

The advantage of this piece of code, is that it’s better readable and maintainable. Get the code: http://bit.ly/2b91GW9

Gotcha

• When you deal with callbacks, (async ), you pass a function in a function and you deal with success and failure.

• The result of an async task (promise) can be fulfilled or rejected. It makes async JS better readable.

• Promises are chainable.

2. Classes

OH I SEE WHAT THE PROBLEM IS

YOU’RE NOT DEFINING CLASSES

Just Syntactical Sugar

Just Syntactical SugarThe class syntax is not introducing a

new object-oriented inheritance model to JavaScript.

They mainly provide more convenient syntax to create old-school constructor functions.

ES5: Class-like definition

var Character

= function (name, type, universe) {this.name = name;

this.type = type;this.universe = universe;

};

Character.prototype.introduce = function () {return "I am " + this.name + ". A " +

this.type + " from " + this.universe + ".";

};

var batman = new Character("Batman", "superhero", "DC"); batman.introduce(); //I am Batman. A superhero from DC.

ES2015: Class-like definitionclass Character {constructor(name, type, universe){this.name = name; this.type = type; this.universe = universe;

}introduce() {return "I am " + this.name + ". A " + this.type + "

from " + this.universe + ".";}}typeof Character; //"function"let batman = new Character("Batman", "superhero", "DC"); batman.introduce(); //I am Batman. A superhero from DC.

Getters & Setters

class SuperHero {constructor(name, ability){ this._name = name; this._ability = ability;

}

setName(name){ this._name = name; }getName(){ return this._name; } setAbility(ability){ this._ability = ability; } getAbility(){ return this._ability; }

}

Get the code: http://bit.ly/2aAdgL4

ES2015: Class inheritance (1)

class Character {constructor(name, type, universe){this.name = name; this.type = type; this.universe = universe;

}

introduce() {return "I am " + this.name + ". A " + this.type

+ " from " + this.universe + ".";}};

ES2015: Class inheritance (2)class Wookiee extends Character {constructor(name){ super(); this.name = name;

this.type = "Wookiee";this.universe = "Star Wars";

}talk(){var roar = "Uughguughhhghghghhhgh raaaaaahhgh"

if(this.name == "Chewbacca")roar = roar + " Millennium Falcon";

return roar;}

};

ES2015: Class inheritance (3)

var chewbacca = new Wookiee("Chewbacca"); chewbacca.introduce();//I am Chewbacca. A wookiee from Star Wars.chewbacca.talk();//Uughguughhhghghghhhgh raaaaaahhgh Millenium Falcon

var chief = new Wookiee("Chief"); chief.introduce();//I am Chief. A wookiee from Star Wars.chief.talk();//Uughguughhhghghghhhgh raaaaaahhgh

Static Methods

Static methods are methods which can be accessed as classname.methodName() instead of classInstanceName.methodName().

ES2015: Static Method (1)class SuperHero {constructor(name, ability){ this._name = name; this._ability = ability;

}

setAbility(name){

this._ability = name; }getAbility(){ return this._ability;}

..

static getMarvelFact() {return "Marvel Comics is the common name

and" + " primary imprint of Marvel Worldwide Inc.";

}}

ES2015: Static Method (2)

//Create an objectvar spiderman = new SuperHero(); spiderman.setName("Spiderman"); spiderman.getMarvelFact();//TypeError: spiderman.getMarvelFact is not a function

//Static Method from ClassNameSuperHero.getMarvelFact(); //Marvel Comics is...

Get the code: http://bit.ly/2arntnv

Gotcha

• ES2015 classes, syntactical sugar, the type is a function under the hood

• Class syntax

• Inheritance: ES2015 classes can extend classes

• Static Classes

1. Modules

ES2015: Modules

Other languages have packages

Before ES2015, JavaScript shared only one global scope which can cause collisions and security problems.

ES2015 Goal: solve the scope problem:No module code executes until requested modules are available and processed.

Modules are JavaScript files that are loaded in a different mode, than scripts.

Here are some differences:

1. Module code automatically runs in strict mode, you can’t opt-out.

2. Variables created in the top level of a module aren’t automatically added to the shared global scope. It only exist in the module.

3. The value of this in the top level of a module is undefined.

4. Modules must export anything that should be available to code outside of the module.

5. Modules may import bindings from other modules.

Example Module (1)libs/leftpad.js

function leftPadFunction (str, len, ch) { str = String(str);

var i = ‐1;if (!ch && ch !== 0) ch = ' '; len = len ‐ str.length;while (++i < len) { str = ch + str;

}return str;}

//export leftPadFunction;module.export {leftpad: leftPadFunction

}

Example Module (2)

app.js

//import * as leftpad from '../leftpad‐module';import leftpad from '../leftpad‐module';

leftpad("lee", 5, "‐") //"‐‐lee"leftpad("9", 3, "0") //"009"

Get the code: http://bit.ly/2aH4QPc

Gotcha

• ES2015 answer to packages

• Fix global scope problem (collisions)

• Modules run in a different mode than script

• this in top level of the module equals undefined

• Modules must export anything that should be available to code outside of the module.

More ECMAScript 2015

More ECMAScript 2015

There are more nice ECMAScript 2015 features which didn’t fit in my presentation top 10:

• Spread Operator

• Maps, Weakmaps, Sets & Weaksets

• Generators

• …

See also: http://bit.ly/2clUmIy

ES2015 & Sencha

Sencha

ES6, TYPESCRIPTSO HOT RIGHT NOW

Not all ES2015 features are new to Ext devs

Some of these new ES2015 features already exist for years in Ext JS, and they are more advanced.

ES2015Tagged Template Templates

ExtExt.XTemplate

Object.is() Ext.Object.equals()Object.assign() Ext.apply()Promises Ext.PromiseClasses Ext.BaseModules Ext.Loader

There’s ES2015 support in Sencha Cmd 6.5• This will require tooling. Our frameworks have to

deal with legacy browsers.

• Sencha will need a transpiler to compile back to old fashioned ES5 code.

• Our goal is to let your ES2015 code run together with the Sencha framework code.

• Cmd is Java based! With Sencha Cmd 6.5, code gets transpiled! Under the hood, we will use Google Closure Compiler. (We replaced Rhino)

Be aware of the transpiler!• If you need to use it, you owe yourself to check the output and end

result

• Transpilers generate code which is supported by the current browsers. It might not be the code what you would expected.

ES2015 support in the next major Ext JS version• Next Ext JS version will make changes in the class system Ext.Base

• Write ES2015 “class” syntax to extend from Sencha classes

• New tooling, Node.js / NPM based with integration of Babel. Will allow TypeScript support.

Resources• http://speakerdeck.com/savelee• http://www.leeboonstra.com• http://es6-features.org/#RawStringAccess

• https://github.com/getify/You-Dont-Know- JS/blob/master/es6 & beyond/README.md#you- dont-know-js-es6--beyond

• https://leanpub.com/understandinges6

• https://developers.google.com/closure/compiler

• https://github.com/google/closure-compiler/wiki/ECMAScript6

• https://kpdecker.github.io/six-speed/

Resources