Loop execution flow in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of a loop in programming?

A loop repeats a block of code multiple times until a condition is met or no longer true.

beginner

What are the three main parts of a for loop in JavaScript?

Initialization (start value), condition (when to stop), and update (how to change the value each time).

intermediate

What happens if the loop condition is always true?

The loop runs forever, causing an infinite loop that can freeze or crash the program.

intermediate

How does a while loop differ from a do...while loop?

A while loop checks the condition before running the code, but do...while runs the code once before checking the condition.

beginner

What is the role of the break statement inside a loop?

It immediately stops the loop and exits it, even if the loop condition is still true.

Which part of a for loop runs only once at the start?

A Body

B Condition

C Initialization

D Update

Initialization sets the starting point and runs once before the loop begins.

What will happen if the condition in a while loop is false at the start?

A The loop will run infinitely

B The loop body will not run at all

C The loop body will run once

D The loop will run twice

A while loop checks the condition first, so if false, it skips the loop body.

What does the continue statement do inside a loop?

A Skips the rest of the current loop iteration and moves to the next

B Stops the loop completely

C Restarts the loop from the beginning

D Exits the program

continue skips the remaining code in the current loop cycle and jumps to the next iteration.

In a do...while loop, when is the condition checked?

A Multiple times before each iteration

B Before the loop runs

C Never

D After the loop runs once

The do...while loop runs the code once, then checks the condition to decide if it repeats.

What is the output of this code?

for(let i = 0; i < 3; i++) {
  if(i === 1) break;
  console.log(i);
}

A 0

B 0 1 2

C 1 2

D No output

The loop stops when i equals 1, so only 0 is printed.

Explain how a for loop executes step-by-step.

Think about the order of the three parts and when the code inside runs.

Describe the difference between while and do...while loops.

Focus on when the condition is checked in each loop.

Practice

1. What does a for loop do in JavaScript?

easy

A. Stops the program immediately

B. Runs code only once

C. Creates a new variable

D. Repeats code a set number of times

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is designed to repeat code multiple times based on a condition.

2. Step 2: Identify the correct description

   Among the options, only repeating code a set number of times matches the for loop behavior.

3. Final Answer:

   Repeats code a set number of times -> Option D

4. Quick Check:

   for loop = repeat fixed times [OK]

Hint: For loops repeat code a fixed number of times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

2. Which of the following is the correct syntax for a for loop in JavaScript?

easy

A. for i = 0 to 5 { }

B. for (let i = 0; i < 5; i++) { }

C. loop (i = 0; i < 5; i++) { }

D. for (i < 5; i = 0; i++) { }

Solution

1. Step 1: Recall JavaScript for loop syntax

   The correct syntax uses for (initialization; condition; increment) { }.

2. Step 2: Compare options to syntax

   Only for (let i = 0; i < 5; i++) { } matches the correct syntax with let i = 0; i < 5; i++.

3. Final Answer:

   for (let i = 0; i < 5; i++) { } -> Option B

4. Quick Check:

   for loop syntax = initialization; condition; increment [OK]

Hint: Remember for loop uses three parts inside parentheses [OK]

Common Mistakes:

• Using incorrect keywords like 'loop'
• Swapping initialization and condition
• Missing semicolons inside for parentheses

3. What will be the output of this code?

let sum = 0;
for (let i = 1; i <= 3; i++) {
  sum += i;
}
console.log(sum);

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Trace the loop iterations

   The loop runs with i = 1, 2, 3. Each time, sum adds i: 0+1=1, 1+2=3, 3+3=6.

2. Step 2: Determine final sum value

   After the loop, sum is 6, which is printed by console.log.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum 1+2+3 = 6 [OK]

Hint: Add numbers step-by-step inside the loop [OK]

Common Mistakes:

• Stopping loop early
• Adding wrong values
• Confusing sum initialization

4. Find the error in this loop:

for (let i = 0; i < 5; i--) {
  console.log(i);
}

medium

A. Console.log is incorrect

B. Syntax error in for loop

C. The loop will run infinitely because i decreases

D. Variable i is not declared

Solution

1. Step 1: Analyze the loop increment/decrement

   The loop uses i--, so i decreases each time starting from 0.

2. Step 2: Check loop condition

   The condition is i < 5, which is true for all i less than 5, including negative numbers. Since i decreases, it never reaches 5 or more, so loop never ends.

3. Final Answer:

   The loop will run infinitely because i decreases -> Option C

4. Quick Check:

   Decrement with < condition causes infinite loop [OK]

Hint: Check if increment matches loop condition direction [OK]

Common Mistakes:

• Confusing i-- with i++
• Ignoring loop condition logic
• Assuming loop stops automatically

5. How many times will this loop run?

let count = 0;
let i = 10;
while (i > 0) {
  i -= 3;
  count++;
}
console.log(count);

hard

A. 4

B. 3

C. 5

D. 6

Solution

1. Step 1: Trace the while loop iterations

   Start i=10, count=0. Each loop: i decreases by 3, count increases by 1.
   Iterations:
   1) i=7, count=1
   2) i=4, count=2
   3) i=1, count=3
   4) i=-2, count=4 (loop stops after this because i > 0 is false)

2. Step 2: Count total iterations

   The loop runs 4 times before i becomes -2 and condition fails.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Subtract 3 until i ≤ 0 counts 4 times [OK]

Hint: Count how many times i decreases by 3 until ≤ 0 [OK]

Common Mistakes:

• Counting one less or more iteration
• Not updating count correctly
• Misunderstanding while loop condition