Statement - Let
let
allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used. This is unlike the var keyword, which defines a variable globally, or locally to an entire function regardless of block scope.
function varTest() { var x = 1; if (true) { var x = 2; // same variable! console.log(x); // 2 } console.log(x); // 2 } function letTest() { let x = 1; if (true) { let x = 2; // different variable console.log(x); // 2 } console.log(x); // 1 } varTest(); letTest();
At the top level of programs and functions, let
, unlike var
, does not create a property on the global object. For example:
var x = 'global'; let y = 'global'; console.log(this.x); // "global" console.log(this.y); // undefined
var Thing; { let privateScope = new WeakMap(); let counter = 0; Thing = function() { this.someProperty = 'foo'; privateScope.set(this, { hidden: ++counter, }); }; Thing.prototype.showPublic = function() { return this.someProperty; }; Thing.prototype.showPrivate = function() { return privateScope.get(this).hidden; }; } console.log(typeof privateScope); // "undefined" var thing = new Thing(); console.log(thing); // Thing {someProperty: "foo"} thing.showPublic(); // "foo" thing.showPrivate(); // 1