JavaScript supports the standard loop constructs you have seen in other programming languages:
let counter = 0;
while (counter < 10) {
console.log(counter);
counter++;
}In a do-while loop, the loop’s body is executed once, and then, if the condition holds, the program returns to the top of the block.
let counter = 10;
do {
console.log(counter);
counter++;
} while (counter < 10);When it comes to counter-controlled loops (when we know exactly how many iterations we need, e.g. when we go over the elements of an array), the for loop provides a more compact syntax:
for (let counter = 0; counter < 10; counter++) {
console.log(counter);
}You can cram in multiple variables, update expressions:
const arr = [10, 20, 30, 40, 50];
for (let i = 0, j = arr.length - 1; i < j; i++, j--) {
const temp = arr[i]
arr[i] = arr[j]
arr[j] = temp
}
console.log(arr);The while and do-while loops are best for event-controlled loops (when the number of iterations is unknown before runtime, e.g., when we read data from a file and don’t know how many lines or how many values are there).
break & continueThe break statement exits out of a loop, while the continue statement will skip to the next iteration:
const arr = [10, 20, 30, 40, 50];
const target = 40;
let index = -1;
for (let i = 0; i < arr.length; i++) {
if (arr[i] === target) {
index = i;
break; // stop early!
}
}
console.log(index); // 3const arr = [10, , 30, , 50];
let count = 0;
let sum = 0;
for (let i = 0; i < arr.length; i++) {
if (arr[i] === undefined) {
continue; // ignore missing data!
}
count++
sum += arr[i]
}
const avg = count === 0 ? 0 : sum / count;
console.log(avg);for..of loopThe for..of loops are used for looping through iterable objects such as arrays.
let arr = [10, 20, 30];
for (let item of arr) {
console.log(item);
}It also works with strings:
const greeting = "Hello!"
for (const ch of greeting) {
console.log(ch);
}for..in loopThe for..in loop allows you to iterate over keys of enumerable object properties.
const user = {
firstName: "Ali",
lastName: "Madooei"
};
for (const key in user) {
console.log(key, user[key]);
}It works with strings and arrays too, but iterates over the indices:
const arr = [10, 11, 12];
for (const key in arr) {
// console.log(key);
console.log(arr[key]);
}const greeting = "Hello!"
for (const key in greeting) {
// console.log(key);
console.log(greeting[key]);
}