The for loop

Learn to write counted loops with for.

The for loop

For tasks that repeat a fixed number of times you use the for loop. The form is for (init; condition; step) { ... }. Init runs once, the condition is tested each iteration, the step is applied after each iteration.

For example for (let i = 1; i <= 5; i++) { console.log(i); } prints numbers from 1 to 5. i++ increases i by one each step. The loop ends when the condition becomes false. Starting the counter at 1 suits a 1-5 range; for arrays you should start at 0.

Counter and step

In a for loop the counter is usually a variable named i. It takes a starting value, the condition checks an upper bound, and the step increases or decreases it. i++ adds one, i-- subtracts one, i += 2 adds two.

Although the counter is traditionally i, nested loops often use j and k. The starting value and step determine how many times the loop runs. For example for (let i = 0; i < 10; i += 2) produces 0, 2, 4, 6, 8.

Exercise Print from 1 to 5

Write a for loop: counter i starts at 1, condition is i <= 5, step is i++. The body calls console.log(i). The output must be five lines: 1, 2, 3, 4, 5.

Show example solution

for (let i = 1; i <= 5; i++) {
    console.log(i);
}

Test yourself

What is the output of for (let i = 1; i <= 3; i++) { console.log(i); }?
```
for (let i = 1; i <= 3; i++) { console.log(i); }
```
- 0 1 2
- 1 2 3
- 1 2 3 4
- 3 2 1
Show answer

1 2 3 — i starts at 1, includes 3; each step adds 1.

Write the for header

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

Show answer

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

Match each part to its job

let i = 0

Init
i < 5

Condition
i++

Step
{ ... }

Loop body

Next lesson: The while loop

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