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:<br>1<br>2<br>3<br>2<br>4<br>6<br>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?<br>

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

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?<br>

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

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.