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Table of Contents
- Variable declaration with const and let
- The scope of let and const
- String Interpolation
- Arrow function
- Default parameters
- Class Expression
- Class Inheritance
- Spread Operator
Variable declaration with const and let
You can use var, let or const to declare a variable.
Let
Variables declared with let can not be redeclared. But you can reassingn a new value.
let name = 'John'
console.log(name) //=> John
name = 'Mike'
console.log(name) //=> Mike
let name = 'Nick' //=> SyntaxError: redeclaration of let nameConst
Variables declared with const can not be redeclared. And you can not reassingn a new value.
const name = 'John'
console.log(name) //=> John
name = 'Mike' //=> TypeError: invalid assignment to const `name'
const name = 'Nick' //=> SyntaxError: redeclaration of let nameThe scope of let and const
The let and const statements declare block scope local variables unlike var.
Example of var
var x = 10
console.log(x) //=>10
{
var x = 5
console.log(x) //=>5
}
console.log(x) //=> 5Example of let
let x = 10
console.log(x) //=>10
{
let x = 5
console.log(x) //=>5
}
console.log(x) //=> 10Example of const
const x = 10
console.log(x) //=> 10
{
const x = 5
console.log(x) //=> 5
}
console.log(x) //=> 10String Interpolation
You can use template literals to read the value of a variable in a string.
let name = 'Mike'
console.log(`I am ${name}`) //=> I am MikeArrow function
Arrow Function was introduced in ES6.
ES5 style function
function greeting(name) {
console.log('Hello' + ' ' + name)
}
greeting('Mike') //=>Hello MikeES6 style function
const greeting = name => {
console.log(`Hello ${name}`)
}
greeting('Mike') //=> Hello MikeDefault parameters
You can assign the default value to an argument.
const add = (x, y = 10) => {
console.log(x + y)
}
add(5) //=> 15Class Expression
The class expressiong is syntactical sugar over JavaScript’s existing prototype-based inheritance.
class Car {
constructor(name) {
this.name = name
}
displayinfo() {
console.log(this.name)
}
}
const car1 = new Car('Honda')
car1.displayinfo() //=> HondaClass Inheritance
Classes can extend one another using extends.
class Car {
constructor(name) {
this.name = name
}
displayinfo(name) {
console.log(this.name)
}
}
class Track extends Car {
constructor(name) {
super()
this.name = name
}
}
track1 = new Track('TL')
track1.displayinfo() //=> TLSpread Operator
Here are some usages of the spread operator.
Spread an array
const arr = [2, 3]
console.log(...arr) //=> 2, 3Combine arrays
const arr = [2, 3]
const arr2 = [1, ...arr, 4, 5]
console.log(arr2) //=> Array(5) [ 1, 2, 3, 4, 5 ]Get multiple arguments as an array
const arr = (arg1, ...args) => {
console.log(arg1, args)
}
arr(1, 2, 3, 4, 5) //=> 1 Array(4) [ 2, 3, 4, 5 ]