JavaScript execution flow - Time & Space Complexity

Time Complexity: JavaScript execution flow

O(n)

Understanding Time Complexity

We want to understand how JavaScript runs code step by step and how long it takes as the code grows.

What happens to the time it takes when we add more instructions or loops?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function printNumbers(n) {
  for (let i = 0; i < n; i++) {
    console.log(i);
  }
}

printNumbers(5);
    

This code prints numbers from 0 up to n-1 using a simple loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that runs console.log repeatedly.
• How many times: It runs exactly n times, once for each number from 0 to n-1.

How Execution Grows With Input

As n grows, the number of times the loop runs grows the same way.

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

Pattern observation: The work grows directly with n; doubling n doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the code grows in a straight line with the input size.

Common Mistake

[X] Wrong: "The loop runs faster because console.log is quick, so time doesn't depend on n."

[OK] Correct: Even if console.log is fast, the loop still runs n times, so more input means more steps.

Interview Connect

Understanding how code runs step by step helps you explain your solutions clearly and shows you know how to think about program speed.

Self-Check

"What if we added a nested loop inside the first loop? How would the time complexity change?"

Practice

1. What happens when JavaScript encounters a function declaration in the code?

easy

A. The function runs immediately.

B. The function is stored but not run until called.

C. The function is ignored.

D. The function causes an error.

Solution

1. Step 1: Understand function declaration behavior

   When JavaScript sees a function declared, it saves it in memory but does not execute it immediately.

2. Step 2: Recognize when functions run

   Functions only run when they are explicitly called later in the code.

3. Final Answer:

   The function is stored but not run until called. -> Option B

4. Quick Check:

   Functions run only when called = A [OK]

Hint: Functions run only when called, not on declaration [OK]

Common Mistakes:

• Thinking functions run immediately on declaration
• Confusing declaration with invocation
• Assuming functions are ignored until called

2. Which of the following is the correct way to call a function named greet in JavaScript?

easy

A. greet();

B. greet;

C. call greet();

D. function greet();

Solution

1. Step 1: Identify function call syntax

   In JavaScript, calling a function requires the function name followed by parentheses.

2. Step 2: Check each option

   greet(); uses greet(); which correctly calls the function. Others are incorrect syntax.

3. Final Answer:

   greet(); -> Option A

4. Quick Check:

   Function call needs parentheses = A [OK]

Hint: Call functions with parentheses: name() [OK]

Common Mistakes:

• Forgetting parentheses when calling functions
• Using 'call' keyword incorrectly
• Confusing declaration syntax with call

3. What will be the output of the following code?

console.log('Start');
setTimeout(() => console.log('Timeout'), 0);
console.log('End');

medium

A. StartEndTimeout

B. EndStartTimeout

C. TimeoutStartEnd

D. StartTimeoutEnd

Solution

1. Step 1: Understand synchronous and asynchronous code

   console.log('Start') runs first and prints 'Start'. setTimeout schedules a function to run later, after current code finishes.

2. Step 2: Trace the execution order

   console.log('End') runs immediately after 'Start'. Then the setTimeout callback runs, printing 'Timeout'.

3. Final Answer:

   Start
   End
   Timeout -> Option A

4. Quick Check:

   Async runs after sync code = D [OK]

Hint: setTimeout runs after current code finishes [OK]

Common Mistakes:

• Assuming setTimeout runs immediately
• Mixing order of console logs
• Ignoring asynchronous behavior

4. Find the error in this code snippet:

function sayHi() {
  console.log('Hi');
}
sayHi

medium

A. console.log syntax error

B. Function name is incorrect

C. Missing parentheses to call the function

D. No error, code runs fine

Solution

1. Step 1: Check function call syntax

   The code writes sayHi without parentheses, so the function is not called.

2. Step 2: Identify the fix

   Adding parentheses sayHi() calls the function and prints 'Hi'.

3. Final Answer:

   Missing parentheses to call the function -> Option C

4. Quick Check:

   Function call needs () = C [OK]

Hint: Always use () to call functions [OK]

Common Mistakes:

• Forgetting parentheses to invoke functions
• Thinking function name alone calls it
• Assuming no error when function not called

5. Consider this code:

console.log('1');
setTimeout(() => console.log('2'), 100);
Promise.resolve().then(() => console.log('3'));
console.log('4');

What is the correct order of output?

hard

A. 1342

B. 4132

C. 1234

D. 1432

Solution

1. Step 1: Understand synchronous, microtask, and macrotask queues

   console.log('1') and console.log('4') run immediately (sync). Promise.then callbacks run next (microtask). setTimeout callbacks run last (macrotask).

2. Step 2: Trace output order

   Output order is '1' (sync), '4' (sync), '3' (Promise microtask), then '2' (setTimeout macrotask).

3. Final Answer:

   1
   4
   3
   2 -> Option D

4. Quick Check:

   Promise microtasks before setTimeout macrotasks = B [OK]

Hint: Promise.then runs before setTimeout even with 0 delay [OK]

Common Mistakes:

• Thinking setTimeout runs before Promise.then
• Ignoring microtask queue priority
• Assuming all async run in order scheduled