Variable and Parameter

Keyword	Scope	IsMutable
var	function	yes
let	block	yes
const	block	no

let

if (true) {
    let a = 40;
    console.log(a); // 40
}
console.log(a);     // undefined.

const

const DOB = '10.05.1998';
DOB = '13.05.1998';             // error, can't reassign the constant

const Languages = ['Python', 'C#'];
Languages = "Java";              // error
Languages.push("JavaScript");    // correct
console.log(Languages);          // ['Python', 'C#', 'JavaScript']

let a = 50; let b = 100;
if (true) {
    let a = 60;
    var c = 10;
    console.log(a/c);         // 6
    console.log(b/c);         // 10
}
console.log(c);                 // 10
console.log(a);                 // 50

Destructuring

Destructuring Arrays

Example

let x = 3;
let y = 2;

[x,y] = [y,x];

//x=2
//x=3 

Reference
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#array_destructuring

Destructuring Objects

Example

let test = {
    firstName: "John",
    lastName: "Doe"
};

let {first, last} = test;

//first = "John"
//last = "Doe"

Reference
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#object_destructuring

default parameters

var doWork = function(name = "John"){
    return name;
};

var result = doWork();
//var result = doWork(undefined);

//result = "John"

//var result = doWork("Scott");
//result = "Scott"

Rest Parameter (is like varargs)

• Rest parameter allows the functions to accept an indefinite number of arguments as as array.

• Rest parameter should appear after any regular parameters.

let doWork = function(name, ...numbers){    //rest parameter
}

function greet(...names){
    names.forEach(name => console.log('Hi, ' + name));
}

greet('Rose', 'Mary', 'Marlo')
//Hi, Rose
//Hi, Mary
//Hi, Marlo

function greet(message, ...names){
    console.log(message);
    names.forEach(name => console.log('Hi, ' + name));
}

greet('Hello, World!', 'Rose', 'Mary', 'Marlo')
//Hello, World!
//Hi, Rose
//Hi, Mary
//Hi, Marlo

Spread Parameter

• Spread parameter is the exact opposite of how Rest parameter works.

• It allows a function to take an array as as argument and then spread out its elements so that they can be assigned to individual parameters.

let doWork = function(x,y,z){
    return x+y+z;
};

var result = doWork(...[1,2,3])    //spread parameter

// x=1  y=2  z=3    result = 6

var a = [4,5,6]
var b = [1,2,3,...a,7,8,9]        //spread parameter

//b = [1,2,3,4,5,6,7,8,9]

function greet(person1, person2){
    console.log('Hi, ' + person1 + ' and ' + person2);    //Hi, John and Jane
}

let names = ['John', 'Jane');
greet(...names);                // spread parameter

Template Literals

let username = "john";
let url = `https:\\github.com\${username}`;

//url = https:\\github.com/john-***********/*-///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

console.log(a);    // error
var a;             // undefined

• JavaScript always hoists the declaration of any variable at the top so even if in code it appears to be after calling the variable it won't give an error.

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