What hoisting is in Javascript - Time & Space Complexity

Time Complexity: What hoisting is

O(1)

Understanding Time Complexity

We want to understand how hoisting affects the order and cost of running JavaScript code.

Specifically, how does hoisting change when and how many times parts of code run?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function example() {
  console.log(a);
  var a = 5;
  console.log(a);
}
example();
    

This code shows variable usage before and after declaration to illustrate hoisting.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The function runs once, with two console.log calls.
• How many times: Each console.log runs once per function call.

How Execution Grows With Input

Since the function runs a fixed number of steps, the work does not grow with input size.

Input Size (n)	Approx. Operations
10	2 console.log calls
100	2 console.log calls
1000	2 console.log calls

Pattern observation: The number of operations stays the same no matter the input size.

Final Time Complexity

Time Complexity: O(1)

This means the code runs in constant time, doing the same amount of work regardless of input.

Common Mistake

[X] Wrong: "Hoisting makes the code run multiple times or loops behind the scenes."

[OK] Correct: Hoisting only moves declarations up before running code; it does not repeat or loop code.

Interview Connect

Understanding hoisting helps you explain how JavaScript runs code step-by-step, a key skill for clear coding and debugging.

Self-Check

"What if we replaced var with let in the example? How would the time complexity change?"

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