Loops: for, while, and forEach

Loops let you repeat a block of code multiple times without rewriting it. JavaScript provides several loop types for different situations.

The for Loop

The classic for loop is the most flexible. It has three parts: initialiser, condition, and update:

for (let i = 0; i < 5; i++) {
  console.log("Iteration " + i);
}
// Outputs: Iteration 0, Iteration 1, Iteration 2, Iteration 3, Iteration 4

Counting backwards:

for (let i = 10; i > 0; i -= 2) {
  console.log(i);  // 10, 8, 6, 4, 2
}

The while Loop

A while loop runs as long as its condition remains true:

let count = 0;

while (count < 3) {
  console.log("count is " + count);
  count++;
}

Use while when you do not know in advance how many iterations you need:

let input = getUserInput();
while (input !== "quit") {
  processInput(input);
  input = getUserInput();
}

Be careful to always update the condition variable inside the loop, or you will create an infinite loop.

The do...while Loop

A do...while loop always executes the body at least once:

let attempts = 0;
do {
  attempts++;
  console.log("Attempt " + attempts);
} while (attempts < 3);

for...of — Iterating Arrays

The for...of loop is the cleanest way to iterate over arrays and other iterables:

const fruits = ["apple", "banana", "cherry"];

for (const fruit of fruits) {
  console.log(fruit);
}
// apple
// banana
// cherry

forEach — Array Method

Arrays have a built-in forEach method that accepts a callback function:

const numbers = [1, 2, 3, 4, 5];

numbers.forEach(function(num) {
  console.log(num * 2);
});

// With an arrow function (more concise):
numbers.forEach(num => console.log(num * 2));

break and continue

break exits the loop immediately. continue skips the rest of the current iteration:

// break example
for (let i = 0; i < 10; i++) {
  if (i === 5) break;
  console.log(i);  // 0, 1, 2, 3, 4
}

// continue example
for (let i = 0; i < 6; i++) {
  if (i % 2 === 0) continue;  // skip even numbers
  console.log(i);  // 1, 3, 5
}

Choosing the Right Loop