Loops
Repeat code with for, for...of, while and array methods.
Loops repeat a block of code. JavaScript offers several kinds; pick the one that reads most clearly.
The classic for loop
JavaScript
for (let i = 1; i <= 5; i++) {
console.log("*".repeat(i));
}Output
* ** *** **** *****
for...of
for...of walks through each item of an array (or the characters of a string) without needing an index.
JavaScript
const cities = ["Delhi", "Chennai", "Kolkata"];
for (const city of cities) {
console.log(`${city} has ${city.length} letters`);
}Output
Delhi has 5 letters Chennai has 7 letters Kolkata has 7 letters
while
JavaScript
let guess = 1;
let attempts = 0;
while (guess * guess < 200) {
guess++;
attempts++;
}
console.log(`${guess} is the first number whose square is at least 200`);
console.log("Attempts:", attempts);Output
15 is the first number whose square is at least 200 Attempts: 14
Array methods instead of loops
Arrays come with methods that loop for you. map transforms every item, filter keeps the items that pass a test, and reduce combines them into one value.
JavaScript
const prices = [120, 45, 300, 80];
const withTax = prices.map((p) => p * 1.05);
const cheap = prices.filter((p) => p < 100);
const total = prices.reduce((sum, p) => sum + p, 0);
console.log(withTax);
console.log(cheap);
console.log("Total:", total);Output
[ 126, 47.25, 315, 84 ] [ 45, 80 ] Total: 545