06_variable.md

Const & let

variable -> use let

1. let is block scope

let height = 172;
console.log(height);
// output: 172
{
    let weight = 58;
    console.log('in the block: ' + weight);
    // output: 'in the block: 58'
}
cosole.log('outside: ' + weight);
// GET SOME ERROR!

2. let can't declare twice (in same scope)

let name = 'Parker';
name = 'Sophie'; // OK here
let name = 'Mike'; 
// SyntaxError: Identifier 'name' has already been declared

compare to var

Scope

function letscope {
    // i is *not* visible out here
    for (let i=0; i<10; i++){
        // i is visible only in the scope
    }
    // i is *not* visible out here
}
function varscope() {
    // i *is* visible out here
    for (var i=0; i<10; i++){
        // i is visible to the whole function
    }
    // i *is* visible out here
}

Redeclare

let weight = 58;
let weight = 666; // SyntaxError
var money = 10000;
var money = 777777777; // It's OK

Global

let nickname = 'hottie';  // globally scoped
var name = 'Parker'; // globally scoped

console.log(window.nickname); // undefined
console.log(window.name); // 'Pakrer'

	let	var
variable	block scope	function scope
redeclaration	SyntaxError	No problem
global window object	not be added	added as properties

constant -> use const

• Enable protection for variables that must remain the same.
• Could improve performance.
• block-scoped

const pi = 3.14;
pi = 15; // TypeError: Assignment to constant variable.

[Note] const in array or object

const arr = ['Parker', 'is'];
console.log(arr); // ['Parker', 'is']

arr.push('handsome!');
console.log(arr); // ['Parker', 'is', 'handsome!']

We won't get any error here. Because in JS, array and object are reference type. We didn't point this constant to something else, it still refers to the same memory location.

const obj = {
    name: 'Teemo'
}
console.log(obj); // {name: 'Teemo'}

obj.skill = 'taunting';
console.log(obj); // {name: 'Teemo', skill: 'taunting'}

If you use object literal to modify the contents of an object, then it's equivalent to creating a new object. That is, it will save the object to another memory location, meaning this constant value has changed.

In the case below, the wrong message will appear (the array is the same)

const obj = {
    name: 'Teemo'
}
console.log(obj); // {name: 'Teemo'}

obj = {
    name: 'Galen'
}
// TypeError: Assignment to constant variable.

const arr = [2, 4, 6];
console.log(arr); // [2, 4, 6]

arr = [1, 3, 5];
// TypeError: Assignment to constant variable.

Conclution

Use let. Don't use var.