How to Write a For Loop in JavaScript

Use a for loop when code needs to repeat a known number of times or move through an array by index. This is the cleanest pattern for counters, index-based updates, and controlled repetition.

What you’ll build or solve

You’ll learn how to write a for loop in JavaScript with the start value, condition, and increment step. You’ll also know how to loop through arrays safely.

When this approach works best

This approach is the right choice when the number of repetitions is known or tied to array length.

Common real-world scenarios include:

• Array iteration
• Countdown timers
• Pagination rendering
• Grid generation
• Repeated DOM updates

This is a bad idea when you only need to loop array values directly. In that case, for...of or array methods may read better.

Prerequisites

You only need:

• A JavaScript file or browser console
• Basic arrays and variables knowledge

Step-by-step instructions

Step 1: Add the start, condition, and increment

A for loop has three parts inside the parentheses.

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

The loop starts at 0, runs while the condition stays true, and increases by 1 after each cycle.

This same structure works well for arrays.

const names = ["Alex", "Mia", "Jordan"];

for (let i = 0; i < names.length; i++) {
  console.log(names[i]);
}

What to look for:

• Start value runs once
• Condition is checked before every cycle
• Increment updates the counter
• i < array.length prevents overflow
• Great for index-based logic

Examples you can copy

Count to 10

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

Loop array

for (let i = 0; i < tasks.length; i++) {
  console.log(tasks[i]);
}

Reverse countdown

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

Common mistakes and how to fix them

Mistake 1: Wrong loop condition

What the reader might do:

for (let i = 0; i <= names.length; i++)

Why it breaks: the final cycle points to an index that does not exist.

Corrected approach:

for (let i = 0; i < names.length; i++)

Mistake 2: Forgetting the increment

What the reader might do:

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

Why it breaks: i never changes, so the loop never ends.

Corrected approach:

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

Mistake 3: Starting at the wrong index

What the reader might do:

for (let i = 1; i < names.length; i++)

Why it breaks: the first array item is skipped.

Corrected approach:

for (let i = 0; i < names.length; i++)

Troubleshooting

If the loop never stops, check the increment step.

If the last item is undefined, switch <= to <.

If the first item is skipped, start at 0.

If the loop only needs values, consider for...of.

Quick recap

• Use for for controlled repetition
• Add start, condition, and increment
• Use < array.length for arrays
• Start at 0 for indexes
• Great for counters and index logic