Known Iterations
Use a for loop when you know how many times you need to run a block.
It combines initialization, condition, and increment in one place.
JavaScript Tutorial
JavaScript for Loop
The for loop is the most common loop when you know exactly how many times to iterate.
It keeps initialization, condition, and step in one concise line.
Why We Need It
Index-based loops are helpful for arrays, pagination, and fixed counts.
A well-structured for loop is easy to read and efficient.
Syntax
for (init; condition; step) { ... }Basic Example
1. Basic for loop
for (let i = 0; i < 3; i++) {
console.log(i);
}A standard for loop counts from 0 to 2.
Real World Example
2. Loop over array
const items = ["a", "b", "c"];
for (let i = 0; i < items.length; i++) {
console.log(items[i]);
}Use indices when you need the position.
Multiple Use Cases
Indices and Counters
for loops are great for index-based iteration over arrays.
Be mindful of the start and end conditions to avoid off-by-one errors.
Nested Loops
Nested loops are useful for grids and combinations, but can be expensive.
Keep the inner work light and avoid unnecessary nesting.
More Examples
Comparison
Without
let i = 0;
console.log(i);
i++;
console.log(i);
i++;With
for (let i = 0; i < 2; i++) {
console.log(i);
}Common Mistakes and Fixes
Off-by-one errors
Double-check loop boundaries (use < vs <= intentionally).
Using length in condition repeatedly
Cache length if needed for performance in hot loops.
Forgetting i++
Ensure the counter changes so the loop can finish.
Mutating array length while looping
Avoid changing the array you are iterating unless necessary.
Interview Questions
What are the three parts of a for loop?
Initialization, condition, and increment (step).
How do you avoid off-by-one errors?
Be explicit about whether the end index is inclusive or exclusive.
When is for...of better?
When you only need the values, not the index.
Practice Problem
Practice: Print the first 10 even numbers using a for loop.
// TODO: print 0 to 18 step 2
One Possible Solution
for (let i = 0; i < 20; i += 2) {
console.log(i);
}Frequently Asked Questions
When should I use for vs for...of?
Use for when you need the index, for...of when you only need values.
Can the increment be negative?
Yes, you can count down with i--.
Is for faster than array methods?
Often similar; choose readability first unless performance is critical.
Try It Yourself
Try changing the loop bounds and step to see how iteration changes.