For loop in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a for loop used for in JavaScript?

A for loop is used to repeat a block of code a specific number of times. It helps automate repetitive tasks.

beginner

Identify the three main parts inside the parentheses of a for loop: for (___; ___; ___)

The three parts are: 1) Initialization (sets a starting point), 2) Condition (checks if the loop should continue), 3) Update (changes the loop variable each time).

beginner

What will this loop print?<br><pre>for (let i = 0; i &lt; 3; i++) { console.log(i); }</pre>

It will print:
0
1
2
Because it starts at 0 and runs while i is less than 3, increasing i by 1 each time.

intermediate

How can you stop a for loop early?

You can use the break statement inside the loop to stop it immediately when a condition is met.

intermediate

What happens if the condition in a for loop is always true?

The loop will run forever, causing an infinite loop, which can freeze or crash your program.

Which part of the for loop controls how many times the loop runs?

A The loop body

B The initialization

C The condition

D The update

The condition is checked before each loop run to decide if the loop should continue.

What does this loop do?

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

A Counts up from 5 to 0

B Counts up from 0 to 5

C Counts down from 0 to 5

D Counts down from 5 to 1

It starts at 5 and decreases i by 1 each time until i is no longer greater than 0.

What keyword stops a for loop immediately?

A break

B stop

C continue

D exit

break exits the loop immediately.

What will happen if you forget to update the loop variable in a for loop?

A The loop will run once

B The loop will run infinitely

C The loop will not run

D The loop will run twice

Without updating, the condition may never become false, causing an infinite loop.

Which of these is a valid for loop header?

A for (let i = 0; i < 5; i++)

B for (i < 5; i++)

C for i = 0; i < 5; i++

D for (let i = 0; i++)

Option A has the correct syntax with initialization, condition, and update inside parentheses.

Explain the three parts of a for loop and how they work together.

Think about how the loop starts, checks if it should continue, and changes each time.

Describe a real-life situation where a for loop could help automate a task.

Imagine doing something many times, like counting items or sending messages.

Practice

1.

What does a for loop do in JavaScript?

easy

A. Changes the value of a variable once

B. Stops the program immediately

C. Repeats a block of code a set number of times

D. Creates a new variable

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: Compare options with the for loop behavior

   Only Repeats a block of code a set number of times correctly describes repeating code multiple times.

3. Final Answer:

   Repeats a block of code a set number of times -> Option C

4. Quick Check:

   For loop = repeat code [OK]

Hint: For loops repeat code multiple times [OK]

Common Mistakes:

• Thinking for loop stops program
• Confusing variable creation with looping
• Assuming for loop runs only once

2.

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

for (___; ___; ___) {
  // code
}

easy

A. for (i = 0; i < 5; i++)

B. for i = 0; i < 5; i++

C. for (i < 5; i = 0; i++)

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

Solution

1. Step 1: Recall the for loop syntax

   The correct syntax is: for (start; condition; change) { ... }

2. Step 2: Check each option against syntax

   for (i = 0; i < 5; i++) matches the correct syntax with initialization, condition, and increment.

3. Final Answer:

   for (i = 0; i < 5; i++) -> Option A

4. Quick Check:

   Syntax = for (init; cond; change) [OK]

Hint: For loop needs three parts inside parentheses [OK]

Common Mistakes:

• Missing parentheses around for loop parts
• Wrong order of parts inside for loop
• Using semicolons incorrectly

3.

What will be the output of this code?

let result = '';
for (let i = 1; i <= 3; i++) {
  result += i + ' ';
}
console.log(result.trim());

medium

A. "3 2 1"

B. "1 2 3"

C. "123"

D. "1 2 3 4"

Solution

1. Step 1: Understand the loop iterations

   The loop runs with i = 1, 2, 3 because i <= 3.

2. Step 2: Track the string concatenation

   Each iteration adds the number and a space to result: "1 ", then "1 2 ", then "1 2 3 ". trim() removes the last space.

3. Final Answer:

   "1 2 3" -> Option B

4. Quick Check:

   Loop 1 to 3 adds numbers with spaces [OK]

Hint: Check loop range and string building carefully [OK]

Common Mistakes:

• Ignoring the spaces added
• Forgetting trim() removes trailing space
• Assuming loop includes 4

4.

Find the error in this for loop code:

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

medium

A. Missing curly braces around loop body

B. Incorrect variable declaration 'let i = 0'

C. Wrong loop condition 'i < 5'

D. Missing increment operator 'i++' in the loop

Solution

1. Step 1: Check the loop increment part

   The loop has 'i' instead of 'i++' which means i never changes.

2. Step 2: Understand the effect of missing increment

   Without increment, the loop runs forever or does not progress, causing an error or infinite loop.

3. Final Answer:

   Missing increment operator 'i++' in the loop -> Option D

4. Quick Check:

   Increment needed to change loop variable [OK]

Hint: Check all three parts of for loop syntax carefully [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Using wrong condition syntax
• Missing braces but allowed in single line

5.

You want to create a for loop that prints only even numbers from 2 to 10 inclusive. Which code snippet does this correctly?

hard

A. for (let i = 2; i <= 10; i += 2) { console.log(i); }

B. for (let i = 0; i <= 10; i++) { if (i % 2 === 0) console.log(i); }

C. for (let i = 2; i < 10; i++) { if (i % 2 === 0) console.log(i); }

D. for (let i = 2; i <= 10; i++) { if (i % 2 === 1) console.log(i); }

Solution

1. Step 1: Identify the range and condition for even numbers

   We want numbers 2, 4, 6, 8, 10 inclusive.

2. Step 2: Check each option for correctness

   for (let i = 2; i <= 10; i += 2) { console.log(i); } increments by 2 starting at 2, printing only even numbers up to 10. for (let i = 0; i <= 10; i++) { if (i % 2 === 0) console.log(i); } starts from 0 and prints 0 which is outside the 2-10 range. for (let i = 2; i < 10; i++) { if (i % 2 === 0) console.log(i); } misses 10 because it uses i < 10. for (let i = 2; i <= 10; i++) { if (i % 2 === 1) console.log(i); } prints odd numbers, which is wrong.

3. Final Answer:

   for (let i = 2; i <= 10; i += 2) { console.log(i); } -> Option A

4. Quick Check:

   Increment by 2 prints evens directly [OK]

Hint: Use i += 2 to skip odd numbers [OK]

Common Mistakes:

• Using wrong loop condition (i < 10)
• Printing odd numbers by mistake
• Not incrementing properly to skip odds