The for loop
- JavaScript course
- Beginner
- Lesson 36/133
- 5 min
Learn the for loop structure: counter, condition and increment steps.
The for loop
The for loop runs a block a fixed number of times. Its header has three parts: init, condition and increment.
for (let i = 1; i <= 3; i++) {
console.log(i);
}Here i is the counter. Each step tests the condition; if true, it runs the block and increments. When false, the loop ends.
Sum and count patterns
The for loop is commonly used for two tasks: accumulating a sum, or walking through a sequence.
Sum example:
let total = 0;
for (let i = 1; i <= 5; i++) {
total = total + i;
}
console.log(total); // 15Here total is the accumulator. Each step adds i to it. When the loop ends, total is 1+2+3+4+5 = 15.
Practice Print 1 to 5
Use a for loop to print the numbers 1 to 5 with console.log.
Starter code
// Print 1 to 5 with console.logSample solution
for (let i = 1; i <= 5; i++) {
console.log(i);
}Practice Sum of numbers 1 to 10
Use a for loop to compute the sum of numbers 1 to 10. Accumulate it in a total variable and finally print it with console.log(total).
Starter code
// Compute the sum here
let total = 0;Sample solution
let total = 0;
for (let i = 1; i <= 10; i++) {
total += i;
}
console.log(total);Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How many times does this loop run?
for (let i = 0; i < 3; i++) {
console.log(i);
}- 2 times
- Forever
- 4 times
- 3 times Correct answer
Explanation: i starts at 0 and runs while less than 3. It runs for 0, 1, 2 — three times.
2 Complete the loop that counts down from 5 to 1
for (let i = 5; i >= ____; i--) { console.log(i); }for (let i = 5; i >= 1; i--) { console.log(i); }Explanation: With 1 the condition becomes i >= 1 and 5, 4, 3, 2, 1 are printed.
3 Match the for loop parts
| Term | Meaning |
|---|---|
let i = 0 |
init |
i < 5 |
condition |
i++ |
increment |
console.log(i) |
body |