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Let / Const

These are a replacement for var.
let has block scoping and const is immutable.

var

function testVar() {
    var a = 30;
    if (true) {
        var a = 50;
        console.log(a);
    }
    console.log(a);
}
testVar(); // 50 50

for (var i = 0; i < 5; i++) {
    console.log(i); // 1, 2, 3, 4
}
console.log(i); // 5

let

function testLet() {
    let a = 30;
    if (true) {
        let a = 50;
        console.log(a);
    }
    console.log(a);
}
testLet(); // 50 30

for (let i = 0; i < 5; i++) {
    console.log(i); // 1, 2, 3, 4
}
console.log(i); // undefined

const

It is not completely immutable. It only prevents the reassignment of a value to a constant, but it doesn't care about modifying the object.

const pi = 3.14;
pi = 42; // TypeError: Assignment to constant variable.

const person = {
    name: "John"
};
person.name = "Mike";
console.log(person.name); // Mike. Doesn't prevent mutation.

Classes

class User {
    // Called during instantiation.
    constructor(username, email, password) {
        (this.username = username),
            (this.email = email),
            (this.password = passwrod);
    }

    // Callable without instantiation.
    static countUser() {
        console.log("There are 50 users");
    }

    // Regular method.
    register() {
        console.log(this.username + " is registered.");
    }
}

// Instantiation.
let bob = new User("Bob", "bob@email.com", "1234");
bob.register(); // Bob is registered.

User.countUser(); // There are 50 users.

Inheritance

class Member extends User {
    constructor(username, email, password, memberPackage) {
        super(username, email, password); // Avoids doing this.username = username...
        this.package = memberPackage; // Needs to be assigned since not in parent class.
    }

    getPackage() {
        console.log(this.username + " uses the " + this.package + " package.");
    }
}

let mike = new Member("Mike", "mike@email.com", "1234", "standard");
mike.getPackage(); // Mike uses the standard package.
mike.register(); // Mike is registered.

Template Literals

Backticks allow multiline strings. Also, we can inject variables and functions with ${ var }.

Before

var name = "John";
var template =
    "<h1>Hello, " + name + "</h1>" + "<p>This is a simple template.</p>";

After

let name = "John";
let template = `<h1>Hello, ${name}</h1>
<p>This is a simple template.</p>`;

Strings

let string = "Hello, I love Javascript.";

string.startsWith("Hello"); // true
string.endsWith("Javascript"); // false, since there is no dot.
string.includes("love"); // true

Numbers

Number.isNaN("cat"); // true
Number.isInteger(1); // true

Set

This is a collection of unique values. If we pass in an array with duplicates, we will get a collection (object) with unique values back. A weakSet is a collection of unique objects only.

var array = [1, 2, 3, 3];

var set = new Set(array); // {1, 2, 3}

// Set methods
set.size; // 3
set.add("cat"); // [1, 2, 3, "cat"]
set.delete(2); // [1, 3, "cat"]

// Convert to array
var array = Array.from(set); // [1, 2, 3]
var array2 = [...set]; // [1, 2, 3]

Arrow Functions

function hello(name) {
    return "Hello" + name;
}
console.log(hello("Jack")); // Hello Jack

// As long as no curly braces are used after the fat arrow, the return is implicit.
var hello = name => "Hello " + name;
console.log(hello("Jack")); // Hello Jack

They don't rebind this, they inherit it.

It provides a shorter syntax, and more importantly, it binds this lexically, avoiding the usage of that or self. Arguments and this inside arrow functions reference their outer function.

Until arrow functions, every new function defined its own this value. An arrow function does not have its own this; the this value of the enclosing execution context is used.

const obj = {
    data: "John",
    getData() {
        console.log(this.data);
    },
    getData2: () => {
        console.log(this.data);
    }
};

obj.getData(); // John
obj.getData2(); // undefined

By not having a this, they are best suited for non-method functions. Using them as methods results in undefined.

// Old
function Person() {
    var that = this;
    that.age = 0;

    setInterval(function growUp() {
        that.age++; // this.age would refer to growUp, instead of Person.
    }, 1000);
}

// New
function Person() {
    this.age = 0;

    setInterval(() => {
        this.age++; // this refers to the Person object.
    }, 1000);
}

For loop

let items = [1, 2, 3];
// Old
for (var i = 0; i < items.length; i++) {
    console.log(array[i]);
}

// New
for (let item of items) {
    console.log(item);
}

Generators

These are functions that can be paused and resumed. It can return i.e. yield values on each pause.

function* generateThreeNumbers() {
    yield 1;
    yield 2;
    yield 3;
}

var numberIterator = generateThreeNumbers();

numberIterator.next(); // { value: 1, done: false }
numberIterator.next(); // { value: 2, done: false }
numberIterator.next(); // { value: 3, done: false }
numberIterator.next(); // { value: undefined, done: true }

another example...

function* generator() {
    console.log("Hello");
    yield "Yield 1 ran...";
    console.log("World");
    yield "Yield 2 ran...";
    return "Done";
}

let gen = generator(); // Set the generator to a variable to extract data.

console.log(gen.next().value); // Hello, Yield 1 ran...
console.log(gen.next().value); // World, Yield 2 ran...
console.log(gen.next().value); // Done...

for (let g of gen) {
    console.log(g); // Hello, Yield 1 ran, World, Yield 2 ran. Ignores last.
}

Destructuring

const address = {
    street: "",
    city: "",
    country: ""
};

// old
const street = address.street;
const city = address.city;
const country = address.country;

// new
const { street, city, country: ctry } = address; // We can also rename the variables

// array
var foo = [1, 2, 3, 4];

var [one, two, three] = foo;
console.log(one); // 1
console.log(two); // 2
console.log(three); // 3

// Default values. They work only if a property is undefined. null, false and 0 are all still values!
var obj = { a: 1 };

var { a, b = "something" } = obj;
console.log(a); // 1
console.log(b); // something

Electron example

// Old
const electron = require("electron");
const app = electron.app;
const BrowserWindow = electron.BrowserWindow;

// New
const electron = require("electron");
const { app, BrowserWindow } = electron;

Spread

Used for copying or deleting properties from one object to another.

Copy

// Without spread
const meal = {
    id: 1,
    description: "Breakfast"
};

const updatedMeal = {
    id: meal.id,
    description: meal.description,
    calories: 600
};

// With spread
const meal = {
    id: 1,
    description: "Breakfast"
};

const updatedMeal = {
    ...meal, // Injects the id and description properties from meal.
    description: "Brunch", // This has precedence over the spread operator.
    calories: 600
};

Delete

const meal = {
    id: 1,
    description: "Breakfast",
    calories: 600
}

const = { id, ...mealWithoutId} = meal; // Destructuring.
console.log(mealWithoutId); // Object without the id property.

Array

const first = [1, 2, 3];
const first = [4, 5, 6];

// old
const combined = first.concat(second);

// new
const combined = [...first, "a", ...second, "b"]; // Easy to add other items.

// Another example
const meals = [
    { id: 1, description: "Breakfast", calories: 420 },
    { id: 2, description: "Lunch", calories: 520 }
];

const meal = {
    id: 3,
    description: "Snack",
    calories: 180
};

const updatedMeals = [...meals, meal]; // Returns a new array with the addition.