Nested loops in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested loop in JavaScript?

A nested loop is a loop inside another loop. The inner loop runs completely every time the outer loop runs once.

beginner

How many times will the inner loop run in this code?<br><pre>for(let i = 0; i < 3; i++) {<br>  for(let j = 0; j < 2; j++) {<br>    console.log(i, j);<br>  }<br>}</pre>

The inner loop runs 2 times for each of the 3 outer loop runs, so 3 × 2 = 6 times total.

beginner

Why use nested loops? Give a real-life example.

Nested loops help when you need to check combinations, like rows and columns in a grid or seats in a theater. For example, checking every seat in every row.

intermediate

What is the output of this nested loop?<br><pre>for(let i = 1; i <= 2; i++) {<br>  for(let j = 1; j <= 3; j++) {<br>    console.log(i * j);<br>  }<br>}</pre>

The output is:
1
2
3
2
4
6
Because for i=1, j runs 1 to 3, printing 1×1, 1×2, 1×3; then for i=2, j runs 1 to 3, printing 2×1, 2×2, 2×3.

intermediate

How can nested loops affect performance?

Nested loops can slow down programs because the total steps multiply. For example, two loops each running 100 times means 10,000 steps. Use them carefully.

What does a nested loop mean in JavaScript?

A Two loops running at the same time

B A loop that runs only once

C A loop inside another loop

D A loop that never ends

A nested loop is when one loop is placed inside another loop, so the inner loop runs completely for each iteration of the outer loop.

How many times will the inner loop run in this code?

for(let i=0; i<4; i++) {
  for(let j=0; j<3; j++) {
    // code
  }
}

A 12 times

B 3 times

C 7 times

D 4 times

The inner loop runs 3 times for each of the 4 outer loop runs, so 4 × 3 = 12 times.

Which real-life example fits nested loops best?

A Turning on a single light bulb

B Checking every seat in every row of a theater

C Counting how many people are in a room

D Writing a single word

Nested loops are great for checking combinations like rows and seats, which is like checking every seat in every row.

What is the output of this code?

for(let i=1; i<=2; i++) {
  for(let j=1; j<=2; j++) {
    console.log(i + j);
  }
}

A 2 3 3 4

B 1 2 2 3

C 3 4 5 6

D 1 1 1 1

The sums are: i=1,j=1 → 2; i=1,j=2 → 3; i=2,j=1 → 3; i=2,j=2 → 4.

Why should you be careful with nested loops?

A They are not allowed in JavaScript

B They always cause errors

C They only work with numbers

D They can make code run slower because steps multiply

Nested loops multiply the number of steps, which can slow down your program if loops run many times.

Explain what nested loops are and give a simple example in JavaScript.

Think about a loop inside another loop and how many times the inner loop runs.

Describe a real-life situation where nested loops would be useful and why.

Think about checking combinations or pairs, like rows and columns.

Practice

1. What does a nested loop in JavaScript mean?
for (let i = 0; i < 3; i++) {
for (let j = 0; j < 2; j++) {
console.log(i, j);
}
}

easy

A. A loop inside another loop, running multiple times

B. A loop that runs only once

C. A loop that runs backwards

D. A loop that skips some numbers

Solution

1. Step 1: Understand the code structure

   The code has one loop inside another, called nested loops.

2. Step 2: Explain the behavior

   The inner loop runs fully for each iteration of the outer loop, so it runs multiple times.

3. Final Answer:

   A loop inside another loop, running multiple times -> Option A

4. Quick Check:

   Nested loop = loop inside loop [OK]

Hint: Nested means one loop inside another [OK]

Common Mistakes:

• Thinking nested loops run only once
• Confusing nested loops with single loops
• Believing nested loops skip numbers

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

easy

A. for (let i = 0; i < 3; i++) { for (let j = 0; j < 2; j++) { } }

B. for i in range(3) { for j in range(2) { } }

C. while (i < 3) { while (j < 2) }

D. loop i from 0 to 3 { loop j from 0 to 2 { } }

Solution

1. Step 1: Identify JavaScript loop syntax

   JavaScript uses for (let variable = start; condition; increment) syntax.

2. Step 2: Check nested loops syntax

   for (let i = 0; i < 3; i++) { for (let j = 0; j < 2; j++) { } } correctly nests two for loops with proper syntax and braces.

3. Final Answer:

   for (let i = 0; i < 3; i++) { for (let j = 0; j < 2; j++) { } } -> Option A

4. Quick Check:

   JS nested loops use for(;;) inside for(;;) [OK]

Hint: Look for 'for (let i=0; ...)' syntax in JavaScript [OK]

Common Mistakes:

• Using Python or other language syntax
• Missing curly braces {}
• Incorrect loop keywords

3. What is the output of this code?

for (let i = 1; i <= 2; i++) {
  for (let j = 1; j <= 3; j++) {
    console.log(i * j);
  }
}

medium

A. 1 2 3 1 2 3

B. 6 5 4 3 2 1

C. 1 4 9 2 5 8

D. 1 2 3 2 4 6

Solution

1. Step 1: Trace outer loop values

   Outer loop i runs 1 and 2.

2. Step 2: Calculate inner loop outputs

   For i=1, j=1,2,3 outputs 1*1=1, 1*2=2, 1*3=3.
   For i=2, j=1,2,3 outputs 2*1=2, 2*2=4, 2*3=6.

3. Final Answer:

   1 2 3 2 4 6 -> Option D

4. Quick Check:

   Multiply i and j in nested loops [OK]

Hint: Multiply outer and inner loop counters [OK]

Common Mistakes:

• Mixing up i and j values
• Not running inner loop fully each time
• Confusing output order

4. Find the error in this nested loop code:

for (let i = 0; i < 3; i++) {
  for (let i = 0; i < 2; i++) {
    console.log(i);
  }
}

medium

A. Outer loop condition is wrong

B. Missing semicolon after for loops

C. Inner loop uses same variable name 'i', causing conflict

D. Console.log syntax is incorrect

Solution

1. Step 1: Check variable names in loops

   Both loops use variable 'i', inner loop redeclares 'i' shadowing outer.

2. Step 2: Explain why this is a problem

   Using same variable name causes confusion and unexpected behavior in nested loops.

3. Final Answer:

   Inner loop uses same variable name 'i', causing conflict -> Option C

4. Quick Check:

   Use different variables for nested loops [OK]

Hint: Use different variable names for each loop [OK]

Common Mistakes:

• Reusing same variable name in nested loops
• Ignoring variable scope issues
• Assuming semicolons are mandatory after for()

5. You want to print a 3x3 grid of stars (*) using nested loops. Which code correctly does this?

hard

A. for (let i = 0; i < 3; i++) { for (let j = 0; j < 3; j++) { console.log('*'); } }

B. for (let i = 0; i < 3; i++) { let row = ''; for (let j = 0; j < 3; j++) { row += '*'; } console.log(row); }

C. for (let i = 0; i < 3; i++) { console.log('***'); }

D. for (let i = 0; i < 3; i++) { let row = '*'; for (let j = 0; j < 3; j++) { row += '*'; } console.log(row); }

Solution

1. Step 1: Understand the goal

   We want 3 rows, each with 3 stars printed together as a line.

2. Step 2: Check each option's output

   for (let i = 0; i < 3; i++) { let row = ''; for (let j = 0; j < 3; j++) { row += '*'; } console.log(row); } builds a string of 3 stars per row and prints it 3 times.
   for (let i = 0; i < 3; i++) { for (let j = 0; j < 3; j++) { console.log('*'); } } prints stars individually, not as rows.
   for (let i = 0; i < 3; i++) { console.log('***'); } prints 3 stars but only 3 times, no loop for columns.
   for (let i = 0; i < 3; i++) { let row = '*'; for (let j = 0; j < 3; j++) { row += '*'; } console.log(row); } starts with '*' and adds three more '*', printing four stars per row.

3. Final Answer:

   for (let i = 0; i < 3; i++) { let row = ''; for (let j = 0; j < 3; j++) { row += '*'; } console.log(row); } -> Option B

4. Quick Check:

   Build row string inside outer loop [OK]

Hint: Build each row string inside outer loop [OK]

Common Mistakes:

• Printing stars one by one without grouping
• Not resetting row string inside outer loop
• Confusing loops for rows and columns