for loops and counting

When the number of repetitions is clear, a for loop is often easier to read than a while loop. It expresses “take values from a sequence one by one” in a single line.

Counting with for

A JavaScript counting loop places initialization, continuation condition, and update in one header, such as for (let i = 0; i < 5; i++).

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

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

Output:

0 1 2 3 4
10 11 12 13 14
1 3 5 7 9

for (initialization; condition; update) initializes once, checks the condition before each iteration, and updates after each iteration.

Fixed-count accumulation

To sum integers from 1 to 100, use for (let i = 1; i <= 100; i++). The condition i <= 100 includes 100.

let total = 0;
for (let i = 1; i <= 100; i++) {
    total += i;
}
console.log(total);

Output:

Sum = 5050

This version makes the value range of i easier to see than an equivalent while loop.

01let total = 0;02for (let i = 1; i <= 100; i++) {03    total += i;04}05console.log(total);

>>>

Fibonacci sequence

The first two Fibonacci numbers are 1, 1. Starting from the third term, each term equals the sum of the previous two terms.

let n = 10;
let first = 1;
let second = 1;
console.log(first + " " + second + " ");
for (let i = 3; i <= n; i++) {
    let current = first + second;
    console.log(current + " ");
    first = second;
    second = current;
}

One possible run:

Enter the number of terms: 10
1 1 2 3 5 8 13 21 34 55

The most important updates are:

first = second;
second = current;

They slide the two-number window forward, so the next loop can keep using the previous two terms.

01let first = 1;02let second = 1;03for (let i = 3; i <= 10; i++) {04    let current = first + second;05    first = second;06    second = current;07}