JavaScript Loops

// Basic for loop: init; condition; update
for (let i = 0; i < 5; i++) {
    console.log(i); // 0, 1, 2, 3, 4
}

// Iterating an array with index
const fruits = ["apple", "banana", "cherry"];
for (let i = 0; i < fruits.length; i++) {
    console.log(`${i}: ${fruits[i]}`);
}
// 0: apple, 1: banana, 2: cherry

// Reverse iteration
for (let i = fruits.length - 1; i >= 0; i--) {
    console.log(fruits[i]); // cherry, banana, apple
}

// Step by 2
for (let i = 0; i <= 10; i += 2) {
    console.log(i); // 0, 2, 4, 6, 8, 10
}

// Nested loops
for (let row = 1; row <= 3; row++) {
    for (let col = 1; col <= 3; col++) {
        process.stdout.write(`(${row},${col}) `);
    }
    console.log();
}

// Arrays - iterates values
const fruits = ["apple", "banana", "cherry"];
for (const fruit of fruits) {
    console.log(fruit); // apple, banana, cherry
}

// With index using entries()
for (const [index, fruit] of fruits.entries()) {
    console.log(`${index}: ${fruit}`);
}
// 0: apple, 1: banana, 2: cherry

// Strings - iterates characters
for (const char of "hello") {
    console.log(char); // h, e, l, l, o
}

// Set - unique values
const set = new Set([1, 2, 3, 2, 1]);
for (const val of set) {
    console.log(val); // 1, 2, 3
}

// Map - key-value pairs
const map = new Map([["a", 1], ["b", 2]]);
for (const [key, value] of map) {
    console.log(`${key} = ${value}`); // a = 1, b = 2
}

// Async iteration with await (sequential)
async function processItems(items) {
    for (const item of items) {
        await processItem(item); // waits for each to complete
    }
}

// for...in on objects
const user = { name: "Alice", age: 28, city: "London" };
for (const key in user) {
    console.log(`${key}: ${user[key]}`);
}
// name: Alice, age: 28, city: London

// Prefer Object.entries for cleaner code
for (const [key, value] of Object.entries(user)) {
    console.log(`${key}: ${value}`);
}

// WARNING: for...in on arrays - avoid!
const arr = ["a", "b", "c"];
for (const key in arr) {
    console.log(key);  // "0", "1", "2" - string indices!
    // Also iterates any inherited or added properties
}

// for...in includes inherited properties
function Person(name) { this.name = name; }
Person.prototype.greet = function() {};
const p = new Person("Alice");
for (const key in p) {
    console.log(key); // "name", "greet" (inherited!)
}
// Check own properties only:
for (const key in p) {
    if (Object.hasOwn(p, key)) console.log(key); // "name" only
}

// while - check BEFORE running body
let count = 0;
while (count < 5) {
    console.log(count); // 0, 1, 2, 3, 4
    count++;
}
// If count starts at 5, body never runs

// Practical: read until a condition is met
let input = "";
while (!isValid(input)) {
    input = getUserInput();
}

// do...while - check AFTER running body (runs at least once)
let attempts = 0;
do {
    console.log(`Attempt ${++attempts}`);
} while (attempts < 3);
// Attempt 1, Attempt 2, Attempt 3

// do...while guarantees one run even if condition starts false
let x = 10;
do {
    console.log("Runs once even though x >= 5"); // runs
} while (x < 5);

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

// forEach - side effects (logging, DOM updates)
nums.forEach((n, index) => {
    console.log(`${index}: ${n}`);
});

// map - transformation, returns new array
const doubled = nums.map(n => n * 2);
console.log(doubled); // [2, 4, 6, 8, 10]
console.log(nums);    // [1, 2, 3, 4, 5] - original unchanged

// forEach vs for...of for async
// WRONG - forEach with async does not await properly
nums.forEach(async n => {
    await fetch(`/api/${n}`); // all fire concurrently, no order
});

// CORRECT - for...of awaits sequentially
for (const n of nums) {
    await fetch(`/api/${n}`); // waits for each one
}

// CORRECT - parallel with Promise.all
await Promise.all(nums.map(async n => fetch(`/api/${n}`)));

// break - exit the loop entirely
const nums = [1, 3, 7, 10, 14, 20];

for (const n of nums) {
    if (n >= 10) {
        console.log(`First number >= 10: ${n}`); // 10
        break;
    }
}

// continue - skip to the next iteration
for (let i = 0; i < 10; i++) {
    if (i % 2 === 0) continue; // skip evens
    console.log(i); // 1, 3, 5, 7, 9
}

// Labeled break - exit outer loop from inner loop
outer: for (let row = 0; row < 3; row++) {
    for (let col = 0; col < 3; col++) {
        if (row === 1 && col === 1) break outer; // exits both loops
        console.log(`${row},${col}`);
    }
}
// 0,0  0,1  0,2  1,0  (stops here)

// break works in switch too (prevents fall-through)
switch (key) {
    case "a": doA(); break;
    case "b": doB(); break;
}