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.

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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!

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