What hoisting is in Javascript - Step-by-Step Execution

Concept Flow - What hoisting is

Start Execution

↓

Scan code for declarations

↓

Move declarations to top

↓

Execute code line by line

↓

Use variables/functions

↓

End Execution

JavaScript first moves all variable and function declarations to the top before running the code, so they can be used earlier than expected.

Execution Sample

Javascript

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

This code shows how variable declaration is hoisted but assignment is not.

Execution Table

Step	Code Line	Action	Variable 'x' State	Output
1	console.log(x);	Print current value of x	undefined	undefined
2	var x = 5;	Declaration hoisted, x initialized as undefined	undefined
3	x = 5;	Assign 5 to x	5
4	console.log(x);	Print current value of x	5	5

💡 All lines executed; variable declaration hoisted but assignment happens in order.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
x	not declared	undefined	5	5

Key Moments - 2 Insights

Why does console.log(x) print undefined before x is assigned 5?

Because the declaration 'var x' is moved to the top and initialized as undefined (see Step 2 in execution_table), but the assignment 'x = 5' happens later (Step 3).

Is the value 5 available before the assignment line?

No, only the declaration is hoisted, not the assignment. So before Step 3, x is undefined.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of the first console.log(x)?

A 5

B undefined

C ReferenceError

D null

💡 Hint

Check Step 1 output in execution_table where x is undefined.

At which step does variable x get the value 5?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Variable x State' column in execution_table for Step 3.

If we change 'var x = 5;' to 'let x = 5;', what would happen at the first console.log(x)?

A Throws ReferenceError

B Prints 5

C Prints undefined

D Prints null

💡 Hint

With 'let', variables are not hoisted as undefined; see JavaScript hoisting rules.

Concept Snapshot

JavaScript hoisting moves declarations to the top before execution.
Variables declared with var are hoisted and initialized as undefined.
Assignments stay in place and run in order.
Accessing var variables before assignment prints undefined.
Functions are fully hoisted and can be called before declaration.
Let and const are hoisted but not initialized, causing ReferenceError if accessed early.

Full Transcript

Hoisting in JavaScript means the interpreter moves all variable and function declarations to the top of their scope before running the code. For variables declared with var, this means they exist from the start but have the value undefined until assigned. For example, in the code, console.log(x) prints undefined before x is assigned 5 because only the declaration is hoisted, not the assignment. This is why the first console.log outputs undefined and the second outputs 5. Variables declared with let or const behave differently and cause errors if accessed before declaration. Understanding hoisting helps avoid confusion about variable values during code execution.

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