JavaScript Array Iteration

Array iteration methods operate on every item in an array. Instead of using traditional for loops, these methods provide a more readable and functional way to process data collections.

1. forEach()

The forEach() method calls a function (callback) once for each array element.

const numbers = [45, 4, 9, 16, 25];
numbers.forEach(myFunction);

function myFunction(value, index, array) {
  console.log(value);
}

2. map()

The map() method creates a new array by performing a function on each array element. It does not change the original array.

const numbers1 = [45, 4, 9, 16, 25];
const numbers2 = numbers1.map(x => x * 2); 
// Result: [90, 8, 18, 32, 50]

3. filter()

The filter() method creates a new array with array elements that pass a "test".

const numbers = [45, 4, 9, 16, 25];
const over18 = numbers.filter(value => value > 18);
// Result: [45, 25]

4. reduce()

The reduce() method runs a function on each array element to produce (reduce it to) a single value. A common use is finding the sum of all numbers.

const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce((total, value) => total + value);
// Result: 99

5. every() and some()

• every(): Checks if all array values pass a test.
• some(): Checks if some array values pass a test.

const numbers = [45, 4, 9, 16, 25];
let allOver18 = numbers.every(x => x > 18); // false
let someOver18 = numbers.some(x => x > 18);   // true

Key Points to Remember

• Iteration methods are cleaner and more modern than for loops
• map() and filter() return new arrays
• forEach() does not return anything (used for side effects)
• reduce() turns an array into a single result (sum, average, object, etc.)
• Arrow functions make iteration code much shorter and easier to read
• None of these methods change the original array (they are non-mutating)