For, While & Do-While Loops

Why We Need Loops

Loops let you repeat a block of code multiple times. Without them, you'd have to write the same code over and over for repetitive tasks. JavaScript has several loop types — choose based on what you're iterating over.

The for Loop

The classic loop. Three parts: initialization, condition, update. Best when you know how many times to loop.

// Basic for loop
for (let i = 0; i < 5; i++) {
  console.log(`Iteration ${i}`);  // 0, 1, 2, 3, 4
}

// Counting backward
for (let i = 10; i >= 0; i--) {
  console.log(i);  // 10, 9, 8... 0
}

// Stepping by 2
for (let i = 0; i <= 20; i += 2) {
  console.log(i);  // 0, 2, 4... 20
}

// Looping through an array by index
const fruits = ["apple", "banana", "cherry"];
for (let i = 0; i < fruits.length; i++) {
  console.log(`${i}: ${fruits[i]}`);
}

// for...of — cleaner for arrays (ES6)
for (const fruit of fruits) {
  console.log(fruit);  // apple, banana, cherry
}

// for...in — for object properties
const person = { name: "Alice", age: 30, city: "NYC" };
for (const key in person) {
  console.log(`${key}: ${person[key]}`);
}

while and do-while

// while — runs while condition is true
let count = 0;
while (count < 5) {
  console.log(count);
  count++;
}

// Real use case: wait for user input or condition
let attempts = 0;
let success = false;
while (!success && attempts < 3) {
  attempts++;
  // simulate trying something
  success = Math.random() > 0.5;
  console.log(`Attempt ${attempts}: ${success ? 'success' : 'fail'}`);
}

// do-while — executes at least once, THEN checks condition
let input;
do {
  input = prompt("Enter a number greater than 0:");
} while (Number(input) <= 0);
// Runs at least once regardless of condition

// Infinite loop guard — always ensure the loop WILL terminate:
// DANGEROUS:
// while(true) {  }  // runs forever!
// SAFE infinite loop with break:
let i = 0;
while (true) {
  if (i >= 10) break;
  console.log(i++);
}

Array Iteration Methods (Better than for loops)

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

// forEach — execute code for each element
numbers.forEach((num, index) => {
  console.log(`${index}: ${num}`);
});

// map — transform each element, returns new array
const doubled = numbers.map(n => n * 2);
// [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

// filter — keep elements that pass a test
const evens = numbers.filter(n => n % 2 === 0);
// [2, 4, 6, 8, 10]

// find — first element that passes a test
const firstBig = numbers.find(n => n > 7);
// 8

// some — does any element pass?
const hasNegative = numbers.some(n => n < 0);  // false

// every — do all elements pass?
const allPositive = numbers.every(n => n > 0); // true

// reduce — collapse to single value
const sum = numbers.reduce((acc, n) => acc + n, 0);  // 55

// Chain methods together!
const result = numbers
  .filter(n => n % 2 === 0)  // [2,4,6,8,10]
  .map(n => n ** 2)           // [4,16,36,64,100]
  .reduce((sum, n) => sum + n, 0); // 220