Home/JavaScript Engine Masterclass 2026/

Hoisting (Critical)

Q: What is Hoisting in JavaScript?

Answer: Hoisting is the behavior where the JavaScript engine moves variable and function declarations to the top of their containing scope during the compilation phase, before the code is executed..

Q: What is the Temporal Dead Zone (TDZ)?

Answer: The TDZ is the period between entering a scope and the actual line where a 'let' or 'const' variable is declared. During this time, the variable cannot be accessed, and trying to do so throws a ReferenceError..

Q: Are function expressions hoisted?

Answer: No. Only function declarations are fully hoisted. Function expressions follow the hoisting rules of the variable they are assigned to (e.g., if using 'const', they are in the TDZ)..

Master this topic with zero to advance depth.

Expert Answer & Key Takeaways

Mastering Hoisting (Critical) is essential for high-fidelity technical architecture and senior engineering roles in 2026.

JavaScript Hoisting

Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope (to the top of the current script or the current function).

1. The Concept of Hoisting

In JavaScript, a variable can be used before it has been declared.

x = 5; // Assign 5 to x
console.log(x);
var x; // Declare x

The JavaScript engine "hoists" the declaration (var x) to the top, so the code effectively becomes:

var x; // Declaration moved to top
x = 5;
console.log(x);

2. The `let` and `const` Keywords

Variables defined with let and const are also hoisted to the top of the block, but not initialized. The block of code is aware of the variable, but it cannot be used until it has been declared. Using a let variable before it is declared will result in a ReferenceError. The variable is in a "Temporal Dead Zone" from the start of the block until it is declared.

carName = "Volvo"; // ReferenceError: Cannot access 'carName' before initialization
let carName;

3. Function Hoisting

Function declarations are hoisted completely, meaning you can call a function before it is defined in the code.

myFunction(); // This works!

function myFunction() {
  console.log("Hello World");
}

[!IMPORTANT] Function expressions (where you assign a function to a variable) are NOT hoisted like declarations. They follow the hoisting rules of the variable keyword (var, let, or const) used to define them.

4. Only Declarations are Hoisted

JavaScript only hoists declarations, not initializations.

var x = 5;
var y = 7;
console.log(x + " " + y); // "5 7"

// vs

var a = 5;
console.log(a + " " + b); // "5 undefined"
var b = 7;

In the second example, only the declaration var b is hoisted, not the value 7. Therefore, b is undefined when the console.log runs.

Hoisting (Critical)

Expert Answer & Key Takeaways

JavaScript Hoisting

1. The Concept of Hoisting

2. The `let` and `const` Keywords

3. Function Hoisting

4. Only Declarations are Hoisted

Top Interview Questions

?Interview Question

?Interview Question

?Interview Question

Course4All Engineering Team

Found an issue or have a suggestion?

Hoisting (Critical)

Expert Answer & Key Takeaways

JavaScript Hoisting

1. The Concept of Hoisting

2. The let and const Keywords

3. Function Hoisting

4. Only Declarations are Hoisted

Top Interview Questions

?Interview Question

?Interview Question

?Interview Question

Course4All Engineering Team

Explore More javascript

Strict Mode

Arrays

Array Methods

Array Search

Found an issue or have a suggestion?

2. The `let` and `const` Keywords