Prefer const

Use const for all references and avoid using var.

ESLint: prefer-const and no-const-assign

This ensures that references are not reassigned, which can lead to bugs and difficult to comprehend code.

Examples

⇣ Incorrect code for this rule:

var snow = 1;
var ice = 2;

⇡ Correct code for this rule:

const snow = 1;
const ice = 2;

Disallow var

Use let instead of var to reassign references.

ESLint: no-var

let is block-scoped rather than function-scoped like var.

Examples

⇣ Incorrect code for this rule:

var snow = 1;
if (true) {
  snow += 1;
}

⇡ Correct code for this rule:

let snow = 1;
if (true) {
  snow += 1;
}

Block Scope

Both let and const are block-scoped. They only exist in the blocks they are defined in.

Examples

{
  let snow = 1;
  const ice = 1;
}
console.log(snow); // ReferenceError: snow is not defined
console.log(ice); // ReferenceError: ice is not defined