Variable hoisting behavior in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is variable hoisting in JavaScript?

Variable hoisting is JavaScript's behavior of moving variable declarations to the top of their containing scope during the compilation phase, before code execution.

intermediate

How does hoisting differ between var, let, and const?

var declarations are hoisted and initialized with undefined. let and const are hoisted but not initialized, causing a 'temporal dead zone' until their declaration is reached.

beginner

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

Accessing a let variable before its declaration throws a ReferenceError because it is in the temporal dead zone until the declaration line is executed.

beginner

Consider this code:

console.log(x); var x = 5;

What is the output and why?

The output is undefined. The declaration var x is hoisted to the top and initialized with undefined, but the assignment = 5 happens after the console.log.

intermediate

Does function declaration hoisting behave the same as variable hoisting?

Function declarations are hoisted completely, meaning the entire function body is moved to the top, allowing you to call the function before its declaration in the code.

What will this code output?

console.log(a); let a = 10;

A 10

B null

C undefined

D ReferenceError

Accessing a let variable before declaration causes a ReferenceError due to the temporal dead zone.

Which keyword's variables are hoisted and initialized with undefined?

A var

B const

C let

D function

var variables are hoisted and initialized with undefined.

What is the temporal dead zone?

A The time after a variable is declared

B A zone where variables are automatically initialized

C The time before a variable is declared where it cannot be accessed

D A special memory area for variables

The temporal dead zone is the period from the start of a scope until a let or const variable is declared, during which accessing it causes an error.

What will this code output?

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

A ReferenceError

B Hello

C undefined

D TypeError

Function declarations are hoisted completely, so foo can be called before its declaration.

Which of these is NOT true about const variables and hoisting?

A <code>const</code> variables can be accessed before declaration without error

B <code>const</code> variables cause a temporal dead zone

C <code>const</code> variables are hoisted but not initialized

D <code>const</code> variables must be initialized at declaration

const variables cannot be accessed before declaration; doing so causes a ReferenceError.

Explain variable hoisting in JavaScript and how it affects var, let, and const declarations.

Think about when variables become available in the code and what happens if you try to use them too early.

Describe how function declarations are hoisted and how this differs from variable hoisting.

Consider what happens when you call a function before it appears in the code.

Practice

1. What happens to a var variable declaration in JavaScript before code execution?

easy

A. It is not moved and must be declared before use.

B. It causes a syntax error if used before declaration.

C. It is moved to the top and initialized as undefined.

D. It is moved to the top and initialized with its assigned value.

Solution

1. Step 1: Understand var hoisting behavior

   JavaScript moves var declarations to the top of their scope and initializes them as undefined.

2. Step 2: Check initialization timing

   The variable is not assigned its value until the code runs the assignment line, so before that it is undefined.

3. Final Answer:

   It is moved to the top and initialized as undefined. -> Option C

4. Quick Check:

   var hoisted = undefined [OK]

Hint: Remember: var is hoisted and set to undefined first [OK]

Common Mistakes:

• Thinking var is not hoisted
• Assuming var is initialized with assigned value immediately
• Confusing var with let or const hoisting

2. Which of the following is the correct way to declare a variable that is hoisted but causes a ReferenceError if accessed before declaration?

easy

A. var x = 5;

B. let x = 5;

C. function x() {}

D. const x;

Solution

1. Step 1: Identify hoisting behavior of let

   let declarations are hoisted but not initialized, so accessing them before declaration causes a ReferenceError.

2. Step 2: Check other options

   var is hoisted and initialized as undefined, so no ReferenceError; const must be initialized at declaration, so const x; is a syntax error; function declarations are hoisted differently.

3. Final Answer:

   let x = 5; -> Option B

4. Quick Check:

   let hoisted but uninitialized causes ReferenceError [OK]

Hint: let and const cause errors if used before declaration [OK]

Common Mistakes:

• Using const without initialization
• Confusing var and let hoisting
• Thinking function declarations behave like let

3. What is the output of the following code?

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

medium

A. undefined 10

B. 10 10

C. ReferenceError undefined

D. undefined undefined

Solution

1. Step 1: Analyze var hoisting for variable a

   The declaration var a is hoisted and initialized as undefined before any code runs.

2. Step 2: Trace console.log outputs

   The first console.log(a) prints undefined because assignment hasn't happened yet. After a = 10, the second console.log(a) prints 10.

3. Final Answer:

   undefined 10 -> Option A

4. Quick Check:

   var hoisted as undefined, then assigned [OK]

Hint: var logs undefined before assignment, then assigned value [OK]

Common Mistakes:

• Expecting ReferenceError on first log
• Thinking var is assigned before declaration
• Confusing let/const behavior with var

4. Find the error in the following code and choose the correct fix:

console.log(b);
let b = 20;

medium

A. Change let to var to avoid ReferenceError.

B. Wrap the code in a function to fix hoisting.

C. Initialize b with a value before logging.

D. Move console.log(b) after the declaration.

Solution

1. Step 1: Identify cause of ReferenceError

   let variables are hoisted but not initialized, so accessing b before declaration causes ReferenceError.

2. Step 2: Fix by reordering code

   Moving console.log(b) after let b = 20; ensures b is declared before use, avoiding the error.

3. Final Answer:

   Move console.log(b) after the declaration. -> Option D

4. Quick Check:

   Access let after declaration to avoid error [OK]

Hint: Use variables only after let/const declaration [OK]

Common Mistakes:

• Changing let to var without understanding scope
• Trying to initialize before declaration (syntax error)
• Assuming wrapping in function fixes hoisting

5. Consider this code:

console.log(x);
if (true) {
  var x = 5;
  let y = 10;
  console.log(y);
}
console.log(x);
console.log(y);

What will be the output and why?

hard

A. undefined 10 5 ReferenceError

B. ReferenceError 10 5 ReferenceError

C. undefined 10 5 10

D. undefined ReferenceError 5 ReferenceError

Solution

1. Step 1: Analyze var x hoisting and scope

   var x is hoisted to the function or global scope and initialized as undefined. So, first console.log(x) prints undefined. After assignment inside the block, x becomes 5.

2. Step 2: Analyze let y block scope and access

   let y = 10 is block scoped. Inside the block, console.log(y) prints 10. Outside the block, console.log(y) causes ReferenceError because y is not defined there.

3. Step 3: Trace remaining console.logs

   After the block, console.log(x) prints 5. The last console.log(y) throws ReferenceError.

4. Final Answer:

   undefined 10 5 ReferenceError -> Option A

5. Quick Check:

   var hoisted globally, let block scoped [OK]

Hint: var is function/global scoped; let is block scoped [OK]

Common Mistakes:

• Assuming let is accessible outside block
• Expecting var to be block scoped
• Confusing initialization timing of var and let