JS - var vs let vs const

2 min readNov 21, 2018

#var

Variables declared using var are always hoisted to the top of their scope.

console.log(j); // ReferenceError: j is not definedconsole.log(i); // undefined
var i = 0;

j is not declared.
var i is hoisted. Here is how the interpreter executed it:

var i;
console.log(i);
i = 0;

#var

var’s are not block-scoped, function-scoped

var i = 0
if (true) {
  var i = 1;
}console.log(i); // 1//function that 'var' defined in 
function foo() {
  var i = 0;
}console.log(i); // ReferenceError: i is not defined

#let

block-scoped
The value of a constant cannot change through reassignment, and it can’t be redeclared.
An initializer(value 값 ) for a constant is required; that is, you must specify its value in the same statement in which it’s declared (which makes sense, given that it can’t be changed later).
선언된 블럭안에서만 사용가능.
선언과 동시에 값도 같이 정해줘야함.
다시 선언할 수 없다.
사용전용 참조를 하지만 그렇다고 변경할수 없는건 아니다. object and array

let i = 0;
if (true) {
  let i = 1;
}console.log(i); // 0

add the practicing code below.
if x,y are declared and defined using ‘const variable’, it throws an error ‘Uncaught TypeError: Assignment to constant variable.’

let {offsetX: x, offsetY: y} = e; //destructuring//const let {offsetX: x, offsetY: y} = e; // throws an errorif(this !== e.target){ 
   x = x + e.target.offsetLeft;
   y = y + e.target.offsetTop;
}
console.log(x, y)

#wait! what is the destruct assignment?

let {offsetX: x, offsetY: y} = e; //destructuring

equal to the code below

let x = e.offsetX, y = e.offsetY;

#const = constant

block scoped.
const i = 0
한번 선언하면 다시 선언을 할 수 가 없다.

const i = 0;
i = 1; // TypeError: Assignment to constant variable.

const doesn’t even let you declare a variable without assigning its (constant) value!

const i; // SyntaxError: Missing initializer in const declaration

const does allow variable mutation (only objects/arrays are mutable in JS).

#ref

JS - var vs let vs const

Written by blossom0417

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