JavaScriptEasy
Explain the concept of "hoisting" in JavaScript
By FrontendPro Editorial Team Updated 8/8/2026
#JavaScript
Answer
Hoisting is a JavaScript mechanism where variable and function declarations are moved ("hoisted") to the top of their containing scope during the compile phase.
- Variable declarations (
var): Declarations are hoisted, but not initializations. The value of the variable isundefinedif accessed before initialization. - Variable declarations (
letandconst): Declarations are hoisted, but not initialized. Accessing them results inReferenceErroruntil the actual declaration is encountered. - Function expressions (
var): Declarations are hoisted, but not initializations. The value of the variable isundefinedif accessed before initialization. - Function declarations (
function): Both declaration and definition are fully hoisted. - Class declarations (
class): Declarations are hoisted, but not initialized. Accessing them results inReferenceErroruntil the actual declaration is encountered. - Import declarations (
import): Declarations are hoisted, and side effects of importing the module are executed before the rest of the code.
The following behavior summarizes the result of accessing the variables before they are declared.
| Declaration | Accessing before declaration |
|---|---|
var foo | undefined |
let foo | ReferenceError |
const foo | ReferenceError |
class Foo | ReferenceError |
var foo = function() { ... } | undefined |
function foo() { ... } | Normal |
import | Normal |