What is the difference between let, const, and var?

Understanding variable declarations in JavaScript.

What You'll Learn

Differences between var, let, const
Scoping rules
Best practices

Comparison

Feature	var	let	const
Scope	Function	Block	Block
Redeclare	Yes	No	No
Reassign	Yes	Yes	No
Hoisting	Yes (undefined)	TDZ	TDZ

Examples

code.jsJavaScript

// var - function scoped
function test() {
  var x = 1;
  if (true) {
    var x = 2; // same variable
  }
  console.log(x); // 2
}

// let - block scoped
function test2() {
  let y = 1;
  if (true) {
    let y = 2; // different variable
  }
  console.log(y); // 1
}

// const - cannot reassign
const z = 1;
z = 2; // Error!

Best Practice

Prefer const by default, use let when reassignment needed, avoid var.

What is a Closure in JavaScript?