Variable hoisting behavior in Javascript - Time & Space Complexity

Time Complexity: Variable hoisting behavior

O(n)

Understanding Time Complexity

Let's explore how variable hoisting affects the speed of running JavaScript code.

We want to see how the number of steps changes when variables are hoisted.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function example(arr) {
  for (let i = 0; i < arr.length; i++) {
    var temp = arr[i] * 2;
    console.log(temp);
  }
}
    

This code loops through an array, doubles each item, and prints it. The variable temp is declared inside the loop using var.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for loop runs through each item in the array.
• How many times: It runs once for every element in the array, so as many times as the array length.

How Execution Grows With Input

As the array gets bigger, the loop runs more times, doing the same steps each time.

Input Size (n)	Approx. Operations
10	About 10 times the steps inside the loop
100	About 100 times the steps inside the loop
1000	About 1000 times the steps inside the loop

Pattern observation: The work grows directly with the size of the input array.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the code grows in a straight line as the input gets bigger.

Common Mistake

[X] Wrong: "Declaring var inside the loop means it runs multiple times and slows down the code a lot."

[OK] Correct: In JavaScript, var declarations are hoisted to the top of the function, so the variable is created once, not repeatedly, which means it does not add extra time per loop.

Interview Connect

Understanding how variable hoisting works helps you explain why some code runs faster or slower, showing you know how JavaScript handles variables behind the scenes.

Self-Check

"What if we changed var to let inside the loop? How would the time complexity change?"

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