The let keyword allows you to declare a variable with block scope.
var x = 10;
// Here x is 10
{
let x = 2;
// Here x is 2
}
// Here x is 10
The const keyword allows you to declare a constant (a JavaScript variable with a constant value). Constants are similar to let variables, except that the value cannot be changed.
var x = 10;
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10
Arrow functions allows a short syntax for writing function expressions. You don't need the function keyword, the return keyword, and the curly brackets.
// ES5
var x = function(x, y) {
return x * y;
}
// ES6
const x = (x, y) => x * y;
const x = (x, y) => { return x * y };
The ... operator expands an iterable (like an array) into more elements:
const q1 = ["Jan", "Feb", "Mar"];
const q2 = ["Apr", "May", "Jun"];
const q3 = ["Jul", "Aug", "Sep"];
const q4 = ["Oct", "Nov", "May"];
const year = [...q1, ...q2, ...q3, ...q4];
The ... operator can be used to expand an iterable into more arguments for function calls:
const numbers = [23,55,21,87,56];
let maxValue = Math.max(...numbers);
The JavaScript for/of statement loops through the values of an iterable objects.
for/of lets you loop over data structures that are iterable such as Arrays, Strings, Maps, NodeLists, and more.
The for/of loop has the following syntax:
for (variable of iterable) {
// code block to be executed
}
variable - For every iteration the value of the next property is assigned to the variable. Variable can be declared with const, let, or var.
iterable - An object that has iterable properties.
const cars = ["BMW", "Volvo", "Mini"];
let text = "";
for (let x of cars) {
text += x + " ";
}
let language = "JavaScript";
let text = "";
for (let x of language) {
text += x + " ";
}
Being able to use an Object as a key is an important Map feature.
const fruits = new Map([
["apples", 500],
["bananas", 300],
["oranges", 200]
]);
Always add a method named constructor():
class ClassName {
constructor() { ... }
}
class Car {
constructor(name, year) {
this.name = name;
this.year = year;
}
}
The example above creates a class named "Car". The class has two initial properties: "name" and "year".