Let and Const in JavaScript with ES6 / ES2015

Two new keywords are available in ES6 / ES2015 to declare variables in JavaScript: let and const. Contrary to var, let and const are block-scoped.

Var has an issue where it’s not block-scoped, which can lead to surprises:

var dog = 'Ralf';

if (true) {
  var dog = 'Skip';
}

console.log(dog); // Skip

Compare this to using let:

let dog = 'Ralf';

if (true) {
  let dog = 'Skip';
}

console.log(dog); // Ralf

Var is properly function-scoped, meaning that the issue doesn’t happen in functions, but in blocks like if or for all bets are off and variables declared with var get hoisted to the parent scope.

Const

With const you can define immutable variables (constants). Trying to re-assign a constant will raise an error:

const PI = 3.1415;

PI = 5; // "TypeError: Assignment to constant variable.

Be careful however, new items can still be pushed into an array constant or added to an object. The following 2 snippets work without complaining because we are not trying to reassign to the variables:

const someArr = [3, 4, 5];

someArr.push(6);

const someObj = {
  dog: 'Skip',
  cat: 'Caramel',
  bird: 'Jack'
};

someObj.camel = 'Bob';

Let vs Var

Many developers now agree that there’s not a very strong case for using var at all anymore and that using let should be the way to go forward.

In short, use let!