What hoisting is in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is hoisting in JavaScript?

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their containing scope before code execution.

beginner

How does hoisting affect variable declarations with var?

Variables declared with var are hoisted and initialized with undefined, so you can use them before their declaration without a ReferenceError, but their value will be undefined until assigned.

intermediate

Are let and const variables hoisted in JavaScript?

Yes, let and const declarations are hoisted but not initialized. Accessing them before declaration causes a ReferenceError due to the 'temporal dead zone'.

beginner

How are function declarations treated during hoisting?

Function declarations are fully hoisted, meaning the entire function is moved to the top of the scope, so you can call the function before its declaration in the code.

intermediate

What happens if you try to use a function expression before it is defined?

Function expressions assigned to variables are hoisted only as variable declarations (with var hoisted as undefined). Calling the function before assignment results in an error because the variable is not yet a function.

What does hoisting do in JavaScript?

A Moves declarations to the top of their scope before execution

B Runs code faster by optimizing loops

C Prevents variables from being used before declaration

D Automatically initializes variables with default values

Hoisting moves variable and function declarations to the top of their scope before the code runs.

What value does a var variable have if accessed before its declaration?

A undefined

B The assigned value

C null

D ReferenceError

var variables are hoisted and initialized with undefined.

What happens if you access a let variable before its declaration?

A Returns undefined

B Returns null

C Throws ReferenceError

D Returns 0

let variables are hoisted but not initialized, causing a ReferenceError if accessed early.

Can you call a function declared with function keyword before its declaration?

A No, it causes a ReferenceError

B Yes, because function declarations are hoisted

C Only if declared with <code>var</code>

D Only if declared with <code>let</code>

Function declarations are fully hoisted, so you can call them before their code line.

What happens if you call a function expression assigned to a var variable before assignment?

A Function runs normally

B Returns undefined

C Throws ReferenceError

D Throws TypeError

The variable is hoisted as undefined, so calling it as a function causes a TypeError.

Explain hoisting in JavaScript and how it affects variables declared with var, let, and const.

Think about where declarations move before code runs and how accessing variables early behaves.

Describe the difference between hoisting of function declarations and function expressions.

Consider how the entire function or just the variable name moves up.

Practice

1. What does hoisting mean in JavaScript?

easy

A. JavaScript converts all variables to global scope automatically.

B. JavaScript moves variable and function declarations to the top before running code.

C. JavaScript delays execution of code until user interaction.

D. JavaScript runs code only after the entire file is loaded.

Solution

1. Step 1: Understand hoisting concept

   Hoisting means JavaScript moves declarations of variables and functions to the top of their scope before running the code.

2. Step 2: Compare options

   Only JavaScript moves variable and function declarations to the top before running code. correctly describes this behavior. Other options describe unrelated behaviors.

3. Final Answer:

   JavaScript moves variable and function declarations to the top before running code. -> Option B

4. Quick Check:

   Hoisting = Moving declarations up [OK]

Hint: Hoisting means declarations move up before code runs [OK]

Common Mistakes:

• Thinking hoisting moves variable assignments, not just declarations
• Confusing hoisting with asynchronous code execution
• Believing hoisting applies to all variable types equally

2. Which of the following variable declarations is hoisted and initialized as undefined?

easy

A. function foo() {}

B. const y = 10;

C. let x = 5;

D. var z = 15;

Solution

1. Step 1: Identify hoisting behavior for variable types

   Variables declared with var are hoisted and initialized as undefined. let and const are hoisted but not initialized, causing a temporal dead zone.

2. Step 2: Match options to behavior

   var z = 15; uses var, so it is hoisted and initialized as undefined. Others do not behave this way.

3. Final Answer:

   var z = 15; -> Option D

4. Quick Check:

   var hoisted as undefined [OK]

Hint: Only var variables are hoisted and set to undefined [OK]

Common Mistakes:

• Assuming let and const behave like var with hoisting
• Confusing function declarations with variable declarations
• Thinking function expressions are hoisted like functions

3. What will be the output of this code?

console.log(myVar);
var myVar = 10;

medium

A. undefined

B. 10

C. ReferenceError

D. null

Solution

1. Step 1: Understand var hoisting with initialization

   The declaration var myVar is hoisted to the top, but the assignment = 10 stays in place.

2. Step 2: Evaluate console.log output

   At the time of console.log(myVar), myVar exists but is undefined because assignment hasn't happened yet.

3. Final Answer:

   undefined -> Option A

4. Quick Check:

   var declaration hoisted, value undefined [OK]

Hint: var declaration hoisted, value assigned later [OK]

Common Mistakes:

• Expecting the value 10 to print immediately
• Confusing ReferenceError with undefined for var
• Thinking let or const behave the same way

4. Find the error in this code related to hoisting:

console.log(foo());
var foo = function() { return 'Hello'; };

medium

A. No error, outputs 'Hello'

B. SyntaxError

C. TypeError: foo is not a function

D. ReferenceError: foo is not defined

Solution

1. Step 1: Understand hoisting of function expressions

   Variable foo is hoisted as undefined, but the function assignment is not hoisted.

2. Step 2: Analyze calling foo before assignment

   Calling foo() before assignment tries to call undefined as a function, causing a TypeError.

3. Final Answer:

   TypeError: foo is not a function -> Option C

4. Quick Check:

   Function expressions not hoisted as functions [OK]

Hint: Function expressions hoisted as undefined, not callable [OK]

Common Mistakes:

• Thinking function expressions are fully hoisted
• Expecting no error because of hoisting
• Confusing ReferenceError with TypeError

5. Consider this code:

console.log(greet());

function greet() {
  return 'Hi!';
}

var greet = function() {
  return 'Hello!';
};

console.log(greet());

What will be the output?

hard

A. 'Hi!' then ReferenceError

B. 'Hello!' then 'Hello!'

C. ReferenceError then 'Hello!'

D. 'Hi!' then 'Hello!'

Solution

1. Step 1: Analyze hoisting of function declaration

   The function declaration function greet() is fully hoisted, so the first greet() call returns 'Hi!'.

2. Step 2: Analyze var assignment after first call

   The var greet declaration is hoisted but initialized as undefined. The assignment to a function expression happens after the first call, so the second greet() call uses the new function returning 'Hello!'.

3. Final Answer:

   'Hi!' then ReferenceError -> Option A

4. Quick Check:

   Function declaration hoisted, var assignment overrides later [OK]

Hint: Function declarations hoisted fully; var assignments override later [OK]

Common Mistakes:

• Thinking var assignment overrides before first call
• Expecting ReferenceError due to var redeclaration
• Confusing function declaration and expression hoisting