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Learn JavaScript: Arrays and Iteration

The .reduce() Method

The .reduce() method iterates through an array and returns a single value.

In the above code example, the .reduce() method will sum up all the elements of the array. It takes a callback function with two parameters (accumulator, currentValue) as arguments. On each iteration, accumulator is the value returned by the last iteration, and the currentValue is the current element. Optionally, a second argument can be passed which acts as the initial value of the accumulator.

const arrayOfNumbers = [1, 2, 3, 4];
const sum = arrayOfNumbers.reduce((accumulator, currentValue) => {
return accumulator + currentValue;
console.log(sum); // 10

The .forEach() Method

The .forEach() method executes a callback function on each of the elements in an array in order.

In the above example code, the callback function containing a console.log() method will be executed 5 times, once for each element.

const numbers = [28, 77, 45, 99, 27];
numbers.forEach(number => {

The .filter() Method

The .filter() method executes a callback function on each element in an array. The callback function for each of the elements must return either true or false. The returned array is a new array with any elements for which the callback function returns true.

In the above code example, the array filteredArray will contain all the elements of randomNumbers but 4.

const randomNumbers = [4, 11, 42, 14, 39];
const filteredArray = randomNumbers.filter(n => {
return n > 5;

The .map() Method

The .map() method executes a callback function on each element in an array. It returns a new array made up of the return values from the callback function.

The original array does not get altered, and the returned array may contain different elements than the original array.

In the example code above, the .map() method is used to add ' joined the contest.' string at the end of each element in the finalParticipants array.

const finalParticipants = ['Taylor', 'Donald', 'Don', 'Natasha', 'Bobby'];
// add string after each final participant
const announcements = => {
return member + ' joined the contest.';

Reverse Loop

A for loop can iterate “in reverse” by initializing the loop variable to the starting value, testing for when the variable hits the ending value, and decrementing (subtracting from) the loop variable at each iteration.

const items = ['apricot', 'banana', 'cherry'];
for (let i = items.length - 1; i >= 0; i -= 1) {
console.log(`${i}. ${items[i]}`);
// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

Do…While Statement

A do...while statement creates a loop that executes a block of code once, checks if a condition is true, and then repeats the loop as long as the condition is true. They are used when you want the code to always execute at least once. The loop ends when the condition evaluates to false.

x = 0
i = 0
do {
x = x + i;
} while (i < 5);
// Prints: 0 1 3 6 10

For Loop

A for loop declares looping instructions, with three important pieces of information separated by semicolons ;:

  • The initialization defines where to begin the loop by declaring (or referencing) the iterator variable
  • The stopping condition determines when to stop looping (when the expression evaluates to false)
  • The iteration statement updates the iterator each time the loop is completed
for (let i = 0; i < 4; i += 1) {
// Output: 0, 1, 2, 3

Looping Through Arrays

An array’s length can be evaluated with the .length property. This is extremely helpful for looping through arrays, as the .length of the array can be used as the stopping condition in the loop.

for (let i = 0; i < array.length; i++){
// Output: Every item in the array

Break Keyword

Within a loop, the break keyword may be used to exit the loop immediately, continuing execution after the loop body.

Here, the break keyword is used to exit the loop when i is greater than 5.

for (let i = 0; i < 99; i += 1) {
if (i > 5) {
// Output: 0 1 2 3 4 5

Nested For Loop

A nested for loop is when a for loop runs inside another for loop.

The inner loop will run all its iterations for each iteration of the outer loop.

for (let outer = 0; outer < 2; outer += 1) {
for (let inner = 0; inner < 3; inner += 1) {


A loop is a programming tool that is used to repeat a set of instructions. Iterate is a generic term that means “to repeat” in the context of loops. A loop will continue to iterate until a specified condition, commonly known as a stopping condition, is met.

While Loop

The while loop creates a loop that is executed as long as a specified condition evaluates to true. The loop will continue to run until the condition evaluates to false. The condition is specified before the loop, and usually, some variable is incremented or altered in the while loop body to determine when the loop should stop.

while (condition) {
// code block to be executed
let i = 0;
while (i < 5) {

Property .length

The .length property of a JavaScript array indicates the number of elements the array contains.

const numbers = [1, 2, 3, 4];
numbers.length // 4


Array elements are arranged by index values, starting at 0 as the first element index. Elements can be accessed by their index using the array name, and the index surrounded by square brackets.

// Accessing an array element
const myArray = [100, 200, 300];
console.log(myArray[0]); // 100
console.log(myArray[1]); // 200
console.log(myArray[2]); // 300

Method .push()

The .push() method of JavaScript arrays can be used to add one or more elements to the end of an array. .push() mutates the original array and returns the new length of the array.

// Adding a single element:
const cart = ['apple', 'orange'];
// Adding multiple elements:
const numbers = [1, 2];
numbers.push(3, 4, 5);

Method .pop()

The .pop() method removes the last element from an array and returns that element.

const ingredients = ['eggs', 'flour', 'chocolate'];
const poppedIngredient = ingredients.pop(); // 'chocolate'
console.log(ingredients); // ['eggs', 'flour']


JavaScript arrays are mutable, meaning that the values they contain can be changed.

Even if they are declared using const, the contents can be manipulated by reassigning internal values or using methods like .push() and .pop().

const names = ['Alice', 'Bob'];
// ['Alice', 'Bob', 'Carl']


Arrays are lists of ordered, stored data. They can hold items that are of any data type. Arrays are created by using square brackets, with individual elements separated by commas.

// An array containing numbers
const numberArray = [0, 1, 2, 3];
// An array containing different data types
const mixedArray = [1, 'chicken', false];

Functions Assigned to Variables

In JavaScript, functions are a data type just as strings, numbers, and arrays are data types. Therefore, functions can be assigned as values to variables, but are different from all other data types because they can be invoked.

let plusFive = (number) => {
return number + 5;
// f is assigned the value of plusFive
let f = plusFive;
plusFive(3); // 8
// Since f has a function value, it can be invoked.
f(9); // 14

Callback Functions

In JavaScript, a callback function is a function that is passed into another function as an argument. This function can then be invoked during the execution of that higher order function (that it is an argument of).

Since, in JavaScript, functions are objects, functions can be passed as arguments.

const isEven = (n) => {
return n % 2 == 0;
let printMsg = (evenFunc, num) => {
const isNumEven = evenFunc(num);
console.log(`The number ${num} is an even number: ${isNumEven}.`)
// Pass in isEven as the callback function
printMsg(isEven, 4);
// Prints: The number 4 is an even number: True.

Higher-Order Functions

In Javascript, functions can be assigned to variables in the same way that strings or arrays can. They can be passed into other functions as parameters or returned from them as well.

A “higher-order function” is a function that accepts functions as parameters and/or returns a function.

JavaScript Functions: First-Class Objects

JavaScript functions are first-class objects. Therefore:

  • They have built-in properties and methods, such as the name property and the .toString() method.
  • Properties and methods can be added to them.
  • They can be passed as arguments and returned from other functions.
  • They can be assigned to variables, array elements, and other objects.
//Assign a function to a variable originalFunc
const originalFunc = (num) => { return num + 2 };
//Re-assign the function to a new variable newFunc
const newFunc = originalFunc;
//Access the function's name property; //'originalFunc'
//Return the function's body as a string
newFunc.toString(); //'(num) => { return num + 2 }'
//Add our own isMathFunction property to the function
newFunc.isMathFunction = true;
//Pass the function as an argument
const functionNameLength = (func) => { return };
functionNameLength(originalFunc); //12
//Return the function
const returnFunc = () => { return newFunc };
returnFunc(); //[Function: originalFunc]

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