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Difference Between let and var
What's the difference between using
let and var in JavaScript?var- variables are scoped to the function body.let- variables are scoped to the immediate enclosing block denoted by{}.
function run() {
var foo = "Foo";
let bar = "Bar";
console.log(foo, bar); // Foo Bar
{
var moo = "Mooo"
let baz = "Bazz";
console.log(moo, baz); // Mooo Bazz
}
console.log(moo); // Mooo
console.log(baz); // ReferenceError
}
run();
In strict mode,
var can be redeclared, while let will raise a SyntaxError.'use strict';
var foo = "foo1";
var foo = "foo2"; // 'foo1' is replaced with 'foo2'.
let bar = "bar1";
let bar = "bar2"; // SyntaxError: Identifier 'bar' has already been declared
var- hoisted and initialized. Meaning they are accessible in their enclosing scope even before they are declared. Their value isundefined.let- hoisted but not initialized until their definition is evaluated. Accessing them before the initialization results in aReferenceError.
function varHoisting() {
console.log(foo); // undefined
var foo = "Foo";
console.log(foo); // Foo
}
function letHoisting() {
console.log(foo); // ReferenceError
let foo = "Foo";
console.log(foo); // Foo
}
var- will create a property on the global object.let- will not create a property on the global object.
var foo = "Foo"; // globally scoped
let bar = "Bar"; // globally scoped but not part of the global object
// in the browser
console.log(window.foo); // Foo
console.log(window.bar); // undefined
// in nodejs
console.log(this.foo); // Foo
console.log(this.bar); // undefined
Last modified 1mo ago