September 1995: LiveScript is released in the Netscape Navigator Browser. It was developed by Brendan Eich.
June 1997: Ecma International releases the first ECMAScript language specification. (note: the European Computer Manufacturers Association was founded in 1961 to standardize computer systems)
December 2009: the ECMAScript 5 standard is released.

From 2016 to today, a new version of ECMAScript is released each year. The language has reached maturity and supports many modern constructs.

Key features of JavaScript:

It is imperative & inspired by C
- if, while, switch, while, do while
It is weakly typed: it has types (String, Number, etc.) but uses implicit cast ("1" - "1" === 0).
It is dynamically typed: types are associated with values rather than expressions (a variable x can be associated with a String, then with a Number).
It supports runtime evaluation with eval. eval('1 === 1') evaluates to true
It supports object orientation with a very powerful prototype-based approach.
While it is not necessarily considered a pure functional language, its functions are first-class citizen. It supports closures and anonymous functions. As a result, it can be used to go pretty far with functional programming patterns.
It is very concise. Newer features such as arrow functions, object and array destructuring leads to very terse code with a very high signal to noise ration.

Quirks

Make sure your target browser supports the feature!

Use caniuse to check what browser support the feature you're using. For instance, for [1, 2].includes(1) requires Array.prototype.includes:

You can then use "polyfills" (or "shims") to support older browser. There are some polyfills for each specific feature, and some other that includes lots of polyfills (e.g. zloirock/core-js).

Polyfills are different from transpiling. Babel is a transpiler, you can see how it work online here:

For instance, it will transpile (notice the replacement of an arrow function):

[1, 2, 3].map((n) => n + 1)

Into:

"use strict";

[1, 2, 3].map(function (n) {
  return n + 1;
});

Undefined everywhere!

var a;
// an uninitialized variable is undefined
console.assert(typeof a === "undefined");

// There are no required arguments in JavaScript
function hello(name) {
  return name;
}

// No raise, will log "undefined"
console.log(hello());

// Here's how to compare to undefined
console.assert(typeof undefined === "undefined");

const anObject = { a: 1 };
// Accessing an absent object key also returns undefined
console.assert(typeof anObject.nonExistent === "undefined");

Printing and interacting with the console

// Do not leave console.log in your code!
// There are linters such as eslint that will check for their absence
console.log("hello");

// In this document, we use assert to show the actual value
console.assert(true === true);

Casting

Rules for string conversion:

String are left as is.
Number are converted to their string representation.
Elements of Array are converted to string, then joined with commas ,.
Objects are converted to [object Object] where Object is the constructor of the object.

Can you guess how those will be converted?

[] + []

{} + {}

"1" + 1

"1" + "1"

"1" - 1

[1, 2] + 2

[1] + 1

function Dog {}

const dog = new Dog()

dog + 1 + "a"

dog - 1

Other gotchas:

// NaN is fun!
console.assert(typeof NaN === "number");
Object.is(NaN, NaN) // true
NaN === NaN // false

A good talk on the topic: Wat

Always use triple comparators (`===`) instead of double (`==`)

// Double equals will coerce values to make them comparable!
console.assert("1" == 1);

// Better
console.assert(!("1" === 1));
console.assert("1" !== 1);

Primitive types vs. reference types

Applied on arrays and objects, == and === will check for object identity, which is almost never what you want.

console.assert({ a: 1 } != { a: 1 });
console.assert({ a: 1 } !== { a: 1 });

const obj = { a: 1 };
const obj2 = obj;
// This is true because obj and obj2 refer to the same object ("identity")
console.assert(obj == obj2);
console.assert(obj === obj2);

Use a library such as lodash to properly compare objects and array

import _ from "lodash";

console.assert(_.isEqual({ a: 1 }, { a: 1 }));
console.assert(_.isEqual([1, 2], [1, 2]));

`Object` and `Array` methods

// Use Object.assign (ES 2015) to copy objects
const target = { a: 1, b: 1};
const source = { b: 2};
const merged = Object.assign(target, source);
console.assert(_.isEqual(merged, {a: 1, b:2});

// Spread operator
const merge2 = {...target, ...source};

// Array.includes (ES7)
const theArray = [1, 2]
console.assert(theArray.includes(1))

Prototypes in JavaScript

JavaScript has a very powerful prototypal inheritance system that is very interesting to study.

The truth is, it is much less used nowadays, and you don't really need to know it to develop with React. It also requires a bit of personal study to fully understand it. So we will leave it aside for now.

The book JavaScript: The Good Parts by Douglas Crockford (2008) is a great introduction to it. Here's a quote from the author:

You make prototype objects, and then … make new instances. Objects are mutable in JavaScript, so we can augment the new instances, giving them new fields and methods. These can then act as prototypes for even newer objects. We don't need classes to make lots of similar objects… Objects inherit from objects. What could be more object oriented than that?

Some good articles:

A Plain English Guide to JavaScript Prototypes

Object literals, assignment and destructuring

Objects

const toaster = { size: 2, color: "red", brand: "NoName" };

// Get ("destructure") one object key
const { size } = toaster;
console.assert(size === 2);

// Note: this also works with functions
function destructuredFunction({ color }) {
  return color;
}

console.assert(destructuredFunction({ color: "red" }) === "red");

// Get the rest with ...rest
const { color, brand, ...rest } = toaster;
console.assert(_.isEqual(rest, { size: 2 }));

// Set default
const { size2 = 3 } = toaster;
console.assert(size2 === 3);

// Rename variables
const { size: size3 } = toaster;
console.assert(size3 === 2);

Enhanced object literals:

const name = "Louis";
const person = { name };
console.assert(_.isEqual(person, { name: "Louis" }));

// Dynamic properties
const person2 = { ["first" + "Name"]: "Olympe" };
console.assert(_.isEqual(person2, { firstName: "Olympe" }));
// Btw, you can include quotes although nobody does this
console.assert(_.isEqual(person2, { firstName: "Olympe" }));

// Short form function

// Before:
const es5Object = {
  say: function () {
    console.log("hello");
  },
};
es5Object.say();

// After: (short form function)
const es6Object = {
  say() {
    console.log("hello");
  },
};
es6Object.say();

Advanced (with prototype):

// Prototype and super()
const firstObject = {
  a: "a",
  hello() {
    return "hello";
  },
};

const secondObject = {
  __proto__: firstObject,
  hello() {
    return super.hello() + " from second object";
  },
};

console.assert(secondObject.hello() === "hello from second object");

Array

const theArray = [1, 2, 3];
const [first, second] = theArray;
const [first1, second2, ...rest] = theArray;

console.assert(first === 1);
console.assert(second === 2);
console.assert(_.isEqualWith(rest, [3]));

`let` and `const`

const constantVar = "a";

// Raises "constantVar" is read-only
constantVar = "b";

let theVar = "a";
theVar = "a";

// Note: const != immutable
const constantObject = { a: 1 };
constantObject.a = 2;
constantObject.b = 3;

// Raises: "constantObject" is read-only
constantObject = { a: 1 };

// const and let are block scoped. A block is enclosed in {} (if, loops, functions, etc.)
{
  const a = "a";
  console.log({ a });
}
// Raises: ReferenceError: a is not defined
console.log({ a });

Note: try to use const as much as you can.

Those variables can't be reassigned. More constraints leads to safer code.
You can't define a const without providing its initial value.
Most people do this in modern JS.

Never use var:

var variables are initialized with undefined, while let and const vars are not initialized and will raise an error if used before definition.
var is globally or function-scoped, depending on whether it is used inside a function.
let and const are block-scoped
let and const cannot be reused for the same variable name

Future of JavaScript: tc39/proposal-record-tuple: ECMAScript proposal for the Record and Tuple value types

Hoisting

See Hoisting on MDN

console.log(typeof variable); // undefined
// console.log(variable); // Raises: ReferenceError: variable is not defined

function hoist() {
  a = 20;
  var b = 100;
}

hoist();

// 20, accessible as a global variable outside of hoist
console.log(a);

// Raises: ReferenceError: b is not defined
// console.log(b);

Arrow functions

The first advantage of arrow function is that they're shorter to write:

// You can define a function this way:
const myFunction = function () {
  console.log("hello world");
};

// With an arrow function, you save a few characters:
const myArrowFunction = () => {
  console.log("hello world");
};

// Some things, like params parentheses, and function code brackets, are optional
const myFunctionToBeShortened = function (a) {
  return a;
};

// Shorter arrow function
const myFunctionToBeShortenedArrowV1 = (a) => {
  return a;
};

// Shortest arrow function
// Remove single param parenthesis, remove function code bracket, remove return
const myFunctionToBeShortenedArrowV2 = (a) => a;
console.assert(myFunctionToBeShortenedArrowV2(1) === 1);

How `this` works in arrow functions

TODO

Best practices

I usually keep the parameters parenthesis. If you add a parameter and weren't including them, you'll have to add them back:

const a1 = (arg) => {};
const a2 = (arg1, arg2) => {};

// vs.
const a3 = (arg) => {};

Classes

class Toaster {
  constructor(color) {
    this.color = color;
  }

  dring() {
    return "dring";
  }
}

// Don't forget new!
// Raises: TypeError: Cannot call a class as a function
// const toaster = Toaster('red');

const toaster = new Toaster("red");
console.log(toaster.dring());

// Inheritance

class BunToaster extends Toaster {
  dring() {
    return super.dring() + " dring";
  }
}

const bunToaster = new BunToaster("red");
console.assert(bunToaster.dring() === "dring dring");

Those are my opinions about other class features:

Avoid using static methods, use plain functions instead.
Avoid using more than one level of inheritance.
Avoid using getter and setters (get and set).
Avoid using classes if you can.

Template literals

const longString = `multi
line
string`;

const name = "Louis";
// Template interpolation
const hello = `Hello ${name}`;

// You can have expressions
const hello1 = `Hello ${name + "!"}`;
const hello2 = `Hello ${name === "Louis" ? name : "Noname"}`;

Template tags

They are used in some libraries, like Apollo and Styled Components.

// First arg is an array of string values, the rest is the expressions

// ["hello ", ""], 3
function templateTag(literals, ...expressions) {
  console.assert(_.isEqual(literals, ["hello ", ""]));
  console.assert(_.isEqual(expressions, [3]));
  return _.join(_.flatten(_.zip(literals, expressions)), "");
}

const result = templateTag`hello ${1 + 2}`;
console.assert(result === "hello 3");

Here's an example with Styled Components:

const Button = styled.a`
  /* This renders the buttons above... Edit me! */
  display: inline-block;
  border-radius: 3px;
  padding: 0.5rem 0;
  margin: 0.5rem 1rem;
  width: 11rem;
  background: transparent;
  color: white;
  border: 2px solid white;

  /* The GitHub button is a primary button
   * edit this to target it specifically! */
  ${props => props.primary && css`
    background: white;
    color: black;
  `}
`

You can see how template tags and arrow functions lead to more concise code!

Loops

`for... of`

Note: prefer using some functional constructs such as map, reduce, etc.

for (const i of [1, 2, 3]) {
  console.log({ i });
}
// 1, 2, 3

for (const key in { a: "aaa", b: "bbb" }) {
  console.log({ key });
}
// 'a', 'b'

Promises

This is only going to be an introduction to the magnificent world of promise.

Async functions (seen later) use promises as a building block.
Promise are indeed async in nature: the calling code continues executing the promise does its thing.
Some Web API return promises, including Fetch

Creating a promise

Note: we use TypeScript in this example, to clarify what's return. You can ignore the type annotations for now.

let isDone: boolean = true

const thePromise = new Promise((resolve, reject) => {
  if (isDone) {
    resolve("the work is done");
  } else {
    reject("this is still pending");
    }
    }

console.assert(thePromise === 'the work is done')

TODO

Consuming a promise

Chaining promises

Async functions

Modules

CommonJS syntax:

const lodash = require("lodash");

ES Module syntax:

import lodash from "lodash";

Imports

// Import all and provide under name
import * as toaster from "./toaster";

// Named import (same as object destructuring!)
import { defaultColor, defaultSize } from "./toaster";

// Renaming imports
import { defaultBrand as toasterDefaultBrand } from "./toaster";

// Default import
import createToaster from "./toaster";

// Import both defaults and other
// import createToaster, {defaultColor} from './toaster'

Exports

In toaster.js:

// Shorthand definition + export
export const defaultSize = 4;

// Alternative export syntax
const defaultBrand = "Moulinex";
export { defaultBrand };

// Default export
const createToaster = ({ size, color }) => ({ size, color });
export default createToaster;

// Note that you have a shorthand default export, but it's not recommended to
// use it as the export won't have a name.
// export default () => ({})

Other features

Optional chaining

let nestedProp = obj.first && obj.first.second;

// with optional chaining:
let nestedProp = obj.first?.second;

Ternary operator

const a = 'a'
const r = a === 'a' ? 'isA' : 'isNotA'

console.assert(r === 'isA')

Self assessment

How old is JavaScript?
Is it a modern language?
Can we say that JavaScript does not have types?
What is the value of this expression: "1" + "1"? 3 + 2 + "5"?
What should I use? let, var, or const?
How do you add support for modern JS features in older browsers?
How are variables scoped in JavaScript?
What should you watch for when comparing variables in JavaScript?
const a = [1]; const b = [1];: what does a == b evaluates to?
How do you write arrow functions?
const {a} = {a: 1}: what does a evaluate to?
How do you write the ternary operator?

Advanced:

How does JavaScript objects "inherit" from each other?

// Write transform1 using a one-line arrow function with object structuring
console.assert(_.isEqual(transform1({name: "Foo"}), {FooA:1}))
console.assert(_.isEqual(transform1({name: "Bar"}), {Bar:1}))

The three things you need:

Use let and const
Object destructuring const { a } = {a: 1}
Arrow functions const noop = () => { }

Other assessments:

References

Future changes:

tc39/proposal-record-tuple: ECMAScript proposal for the Record and Tuple value types.

20 KiB Raw Blame History Unescape Escape

Table of Contents

Modern JavaScript

Objectives of this course

Prerequisites

Check your knowledge of JS

Introduction