JavaScript Hoisting Cheat Sheet
What is Hoisting?
Hoisting is JavaScript’s default behavior of moving declarations to the top of the current scope (script or function).
Key Points
Only declarations are hoisted, not initializations.
Both variable and function declarations are hoisted.
Function expressions are not hoisted.
let and const declarations are hoisted but not initialized.
Variable Hoisting
var Declarations
var variables are hoisted to the top of their function scope.
They are initialized with undefined.
var x = 5;
let and const Declarations
let and const variables are hoisted to the top of their block scope.
They are not initialized; accessing them before initialization results in a ReferenceError.
let y = 10;
console.log(z); // ReferenceError
const z = 15;
Function Hoisting
Function Declarations
Function declarations are hoisted to the top of their scope.
They are fully hoisted, including their definition.
function hoistedFunction() {
console.log(“Hello, World!“);
}
Function Expressions
Function expressions are not hoisted.
They are treated like variable assignments and are initialized only when the execution reaches that line.
var nonHoistedFunction = function() {
console.log(“Hello, World!“);
};
Arrow Functions
Arrow functions behave like function expressions and are not hoisted.
var arrowFunction = () => {
console.log(“Hello, Arrow!“);
};
Class Hoisting
Classes are hoisted but not initialized.
Accessing a class before its declaration results in a ReferenceError.
class MyClass {
constructor() {
console.log(“Class created“);
}
}
Temporal Dead Zone (TDZ)
The TDZ is the time between entering scope and variable declaration where variables are in an uninitialized state.
Accessing a variable in the TDZ results in a ReferenceError.
console.log(a); // ReferenceError: Cannot access ‘a’ before initialization
let a = 3;
}
Best Practices
Declare all variables at the top of their scope to avoid confusion.
Use let and const instead of var to prevent unexpected behavior.
Be mindful of the TDZ when using let and const.
