JavaScript Basics

The Complete Guide to Using Arrays in JavaScript

Arrays are lists of objects that can be manipulated in various ways. Each entry can be accessed by their own index. Arrays can be combined in various ways and they can be also be nested in each other, letting us create multi-dimensional arrays. We can make arrays out of a collection of any objects. They can also be destructed into variables so each entry can be accessed and manipulated individually. JavaScript arrays are zero-indexed so the starting index of each array is always zero. This means that index 0 has the first element of the array.

Arrays can contain any type of object. They do not have to be the same type of objects. Also if an array entry with a given index hasn’t been assigned yet, it has an undefined value.

Examples of things that can be stored with arrays include:

  • to-do list
  • recipe list
  • address book contacts
  • shopping list
  • grocery list
  • your appointments
  • anything else that can be entered into a list.

Without arrays, all we can do is declare variables for each entry individually which is a real pain and not practical.

Declaring Arrays

To declare arrays, we use the let or const keyword — like this:

let arr = [1,2,3];
const arr2 = [1,2,3];

We use let for variables and const for arrays that don’t change.

Also, we can declare the arrays first and then insert the values later, like this:

let arr = []
arr[0] = 1;
arr[1] = 2;
arr[2] = 3;

This is the same as let arr = [1,2,3] since we have the same entries and the same order in both arr arrays.

We can declare arrays with different type of values in the same array, like this:

let arr = [1, 'chicken', {foo: 'bar'}];

As you can see, it doesn’t matter what kind of data we put in the array. However, we do have to be careful to avoid data type errors when we traverse or manipulate arrays by checking the type and content of the objects, and whether any entry is null or undefined .

Another way to create an array is the new keyword, like this:

let names = new Array('Bob','Jane', 'John');

Accessing Array Data

We access array items by their index. For example, to get the second element of an array, we write:


If arr is assigned to [1, ‘chicken’, {foo: ‘bar’}]; , then arr[1] would be 'chicken' .

Get Array Size

Array is an object that has the length property that we can get the size of the array, so the names array we have above would have a length of three. We write the names.length to access the length of the array.

Multidimensional Arrays

In JavaScript, multidimensional arrays are just an array nested in another array. So, to declare a multidimensional array, we can use the same declaration methods as above, except we replace the entities inside with arrays. For example, we can write:

let multiArray = [[1,2,3], [4,5,6]];


let multiArray = new Array([1,2,3], [4,5,6]);

We can also write:

let multiArray = [];
multiArray[0] = [];
multiArray[0][0] = 1;
multiArray[0][1] = 2;
multiArray[0][2] = 3;
multiArray[1] = [];
multiArray[1][0] = 4;
multiArray[1][1] = 5;
multiArray[1][2] = 6;

All three of these pieces of code are equivalent.

We access multidimensional array entries by adding another square bracket with the index of the inner array. For example, if we want to get the second item of the first array in multiArray then we write:


Traversing Arrays

We can traverse the values of arrays with loops. Loops are pieces of code that repeat until the ending condition is met. In JavaScript, we have the for loop, while loop and the do...while loop.

For Loop

With the for loop, given that we have the name array, we can traverse the loop by running:

let names = new Array('Bob','Jane', 'John');
for (let i = 0; i < names.length; i++){

A for loop is usually written with the starting condition as the first statement, then the ending condition in the second statement, and the index changing condition in the third statement. The statements are separated by the semicolons. Because arrays start with index zero in JavaScript, we terminate the loop when the array’s index reaches one less than the array’s length.

While Loop

while loop will loop whenever a condition stays true.

For example, the loop below will run if the index number i is less than three:

const array = [1,2,3];let i = 0;
while(i < array.length){

If the condition in the parentheses is never true, then the loop content will never run.

Do While Loop

do...while loop will always execute the first iteration.

const array = [1,2,3];let i = 0;
while(i < array.length)

In the example above, it will at least log 0, but it will also log 1 and 2 in this case since those two numbers are less than three.

With the above loops, you can call break to stop the loop or return before the loop is completely finished.

//do while loop
const array = [1,2,3];

let i = 0;

  if (i == 1}{    
while(i < array.length)

//while loop
i = 0;
while(i < array.length){  
  if (i == 1){ 

//for loop
for (let j = 0; j < array.length; j++){  
  if (j == 1){

In the above examples, you will not see two logged.

An example of returning from within the loop:

const loop = ()=>{
  const array = [1,2,3];
  for (let j = 0; j < array.length; j++){
    if (j == 1){
      return j;
loop() //returns 1

You can also skip iterations with the continue statement:

const array = [1,2,3];
for (let j = 0; j < array.length; j++){  
  if (j == 1){
  console.log(j) // 1 will be skipped;

Array Properties

Since an array is a JavaScript object. It is its own properties. Like any object in JavaScript, it has the prototype property which lets developers add methods and properties of an array object. The constructor property is the reference to the function that’s created by the array object’s prototype. The length returns the size of the array.

Array Methods

To make manipulating arrays easy, the JavaScript’s standard library contains lots of array methods that make manipulating arrays easy. There are methods to find and filter items, and add and remove items in an array for example. There are also functions to combine multiple arrays into one.

These are some common array methods that you can use to do common operations with arrays.


forEach will iterate through every entry of the array. You cannot break out of it or return a value from it. It takes a callback function where you can execute code.


const array = [1,2,3];
array.forEach(a => {  

In the above example, all the numbers in the array will be logged.


Array.find will return the element in the array with the given condition. For example, if you want to get certain numbers from the array, you do this:

const array = [1,2,3];
const num = array.find(a => a == 2); // returns 2

find returns a single result.


Array.findIndex will return the index of the element in the array with the given condition. It takes a callback function that returns a given condition. For example, if you want to get the index of a certain number from the array, you do this:

const array = [1,2,3];
const num = array.findIndex(a => a == 2); // returns 1


Array.filter will return an array of items that meet the given condition. It takes a callback function that returns a given condition. filter returns a new array.

For example, if you want to get the index of a certain number from the array, you do:

const array = [1,2,3];
const numArray = array.filter(a => a == 2); // returns [2]


Array.includes checks if an item exists in an array. It takes a number or string which the function can compare.

const array = [1,2,3];
const includesTwo = array.includes(2); // returns true


Array.somechecks if some items meet the condition given. It takes a callback function which returns a boolean for the condition.

const array = [1,2,3];
const includesTwo = array.some(a => a == 2); // returns true
const includesFive = array.some(a => a == 5); // returns false


Array.every checks if every item meets the condition given. It takes a callback function which returns a boolean for the condition.

const array = [1,2,3];
const everyElementIsTwo = array.every(a => a == 2); // returns false
const everyElementIsNumber = array.every(a => typeof a == 'number'); // returns true since every item in the array is a number


Array.isArray checks if an object given is an array. It is a convenient way to check if an element is an array.

const array = [1,2,3];const notArray = {};
let objIsArray = Array.isArray(array); // true
objIsArray = Array.isArray(notArray); // false

Array.from(new Set(array))

Set is an object that cannot have duplicate entries. You can create a new Setfrom an array then convert it back to an array.

const array = [1,2,2,3];
const arrayWithDups = Array.from(new Set(array)); //returns new array without duplicates, [1,2,3]

Array.slice(startIndex, endIndex)

Returns a new array from startIndex to endIndex — 1 .


const arr = [1,2,3,4,5];
const newArr = arr.slice(0,2);
console.log(newArr); // returns [1,2]

Array.splice(index, numberOfItems)

Remove array item in place with the given index, and then numberOfItems after it.

For example:

const arr = [1,2,3,4,5];
console.log(arr); // returns [3, 4, 5] since we specified that we remove item located at index 0 and 2 items after that.


Array.sort sorts array in place according to the condition you return insortFunction .

The sortFunction should be in this format:

const sortFunction = (a, b) {
  if (a < b) {
    return -1;
  if (a > b) {
    return 1;
  // a must be equal to b
  return 0;

By default, if no sortFunction is specified, then the array items will be converted to a string and will be sorted according to the Unicode value of the string.

Array.fill(newElement, startIndex, endIndex)

Array.fill will add or replace the element with the element specified from startIndex to endIndex . If no startIndex is defined then it will start from 0. If no endIndex is defined, then it will change values up to the end of the array.

For example:

let array = [1, 2, 3, 4, 5];console.log(array.fill(0, 2, 4));
// array is now [1, 2, 0, 0, 0]console.log(array.fill(5, 1));
// array is now [1, 5, 5, 5, 5]console.log(array.fill(6));
// array is now [6, 6, 6, 6, 6]

Recursively Flatten Array

Array.flat function does not do a good job of recursively flatten arrays. The depth is limited and it does not flatten all kinds of nested array structures. The better way to do it is to write a recursive function to do it.

let arr1 = [1, 2, [3, 4], 5];
let arr2 = [1, 2, [3, 4], [5, [6,[7,]]]];const flatten = (items) => {
  const flat = [];  items.forEach(item => {
    if (Array.isArray(item)) {
    } else {
  });  return flat;


Array.join will return a string by concatenating the entries after they are converted to string with separator between each entry. Works best with string and number arrays.


const arr = [1,2,3];
console.log(arr.join(',')) // get '1,2,3'const arr = ['1',2,3];
console.log(arr.join(',')) // get '1,2,3'


Array.indexOfwill return the first index of the elementToFind in the array. Works best with string and number arrays. If you want to find a non-string or number object, use Array.findIndex . Returns -1 is element is not found.


const arr = [1,2,3];
console.log(arr.indexOf(1)); // returns 0const arr2 = [1,1,2,3];
console.log(arr2.indexOf(1)) // still 0


Array.lastIndexOfwill return the last index of the elementToFind in the array. Works best with string and number arrays. Returns -1 is element is not found. The function also takes a starting index to start searching backward as a second parameter


const arr = [1,2,3];
console.log(arr.indexOf(1)); // returns 0const arr2 = [1,1,2,3];
console.log(arr2.indexOf(1)) // returns 1const arr3 = [3,1,2,3]
console.log(arr3.lastIndexOf(3, 2)) // returns 0, start searching backwards from index 2


Array.push adds a new element to an array.


let arr = [1,2,3];
console.log(arr) // [1,2,3,4]


Array.pop removes the last element of the array.


let arr = [1,2,3,4];
console.log(arr) // [1,2,3] returns a new array which transforms the existing array’s element by calling the mapFunction . mapFunction takes one argument, which is the array element.


let arr = [1,2,3,4];
const newArr =>a*2)
console.log(newArr) // [2,4,6,8]


Array.reduce combines elements of an array to return a value by calling the reduceFunction on all the elements to combine them. reduceFunction takes two arguments, which is the current and next element in the array.


const arr = [1,2,3,4];
const sum = arr.reduce((a,b)=>a+b);
console.log(sum); // returns 10


Array.reverse returns a new array with the existing array’s elements in reverse order.


const arr = [1,2,3,4];
console.log(arr.reverse()) // [4,3,2,1]

Now that we learned about arrays, we can do a lot more in our JavaScript programs. There’s a big world ahead of us!

JavaScript Basics

JavaScript Cheat Sheet — JSON, Loops, and Promises

JavaScript is one of the most popular programming languages for web programming.

In this article, we’ll look at the basic syntax of modern JavaScript


We can create JSON strings from JavaScript objects with the JSON.stringify method:

const obj = {
  "name": "Jane",
  "age": 18,
  "city": "Chicago"
const json = JSON.stringify(obj);

And we can convert JSON strings back to JavaScript objects with JSON.parse :

const obj = JSON.parse(json);

We can use it to store data in local storage.

We’ve to convert objects into strings first to store them.

To store objects we call localStorage.setItem :

const obj = {
  "name": "Jane",
  "age": 18,
  "city": "Chicago"
const json = JSON.stringify(obj);
`localStorage.setItem('json', json);`

The first argument is the key.

And we can get data by their keys with getItem :



JavaScript comes with various kinds of loops.

One kind of loop is the for loop:

for (let i = 0; i < 10; i++) {

We can loop through any kind of iterable object with the for-of loop:

for (let i of custOrder) {

Some iterable objects include arrays, strings, and node lists.

Another kind of loop is the while loop:

let i = 1;
while (i < 100) {
  i *= 2;

There’s also the do-while loop:

let i = 1;
do {
  i *= 2;
} while (i < 100)

The break keyword lets us end the loop early:

for (let i = 0; i < 10; i++) {
  if (i == 5) {

The continue keyword lets us skip to the next iteration:

for (let i = 0; i < 10; i++) {
  if (i == 5) {

Data Types

JavaScript comes with various data types.

They include:

let age = 18;                           // number
let name = "Jane";                      // string
let name = { first: "Jane", last: "Doe" };  // object
let truth = false;                      // boolean
let sheets = ["HTML", "CSS", "JS"];       // array
let a; typeof a;                        // undefined
let a = null;                           // value null


We can create an object with curly braces:

let student = {
  firstName: "Bob",
  lastName: "Doe",
  age: 18,
  height: 170,
  fullName() {
    return `${this.firstName} ${this.lastName}`;

It has properties and methods.

this is the student object itself.

We can call fullName with student.fullName() .

And we can assign values to properties with:

student.age = 19;


We can create promises with the Promise constructor:

function add(a, b) {
  return Promise((resolve, reject) => {
    setTimeout(() => {
      if (typeof a !== "number" || typeof b !== "number") {
        return reject(new TypeError("Inputs must be numbers"));
      resolve(a + b);
    }, 1000);

We can resolve to fulfill the promise with the sum.

And we call reject to reject the promise with an error.

Then we can call then to get the resolved value and catch to get the error values:

const p = sum(10, 5);
p.then((result) => {
}).catch((err) => {


We can work with JSON, local storage, promises, and loops with JavaScript.

JavaScript Basics

JavaScript Cheat Sheet — Numbers, Strings, and Regex

JavaScript is one of the most popular programming languages for web programming.

In this article, we’ll look at the basic syntax of modern JavaScript.


The toFixed method lets us round a number:

(3.14).toFixed(0);  // returns 3

The toPrecision method lets us round a number:

(3.14).`toPrecision`(1);  // returns 3.1

The valueOf method returns a number:


The Number function lets us convert anything to a number:


parseInt converts non-numeric values to an integer:

parseInt("3 months");

parseFloat converts non-numeric values to a floating-point number:

parseFloat("3.5 days");

The Number function also comes with some constant properties.

They include:

  • Number.MAX_VALUE — largest possible JS number
  • Number.MIN_VALUE — smallest possible JS number
  • Number.NEGATIVE_INFINITY —  negative infinity
  • Number.POSITIVE_INFINITY — positive infinity


We can do various mathematical operations with the Math object.

Math.round rounds a number to an integer:


Math.pow raises a base to an exponent:

Math.pow(2, 8)

Math.sqrt takes the square root of a number:


Math.abs takes the absolute value of a number:


Math.ceil takes the ceiling of a number:


Math.floor takes the floor of a number:


Math.sin takes the sine of a number:


Math.cos takes the cosine of a number:


Math.min returns the minimum number in the list:

Math.min(1, 2, 3)

Math.max returns the max number in the list:

Math.max(1, 2, 3)

Math.log takes the natural log of a number:


Math.exp raises e to the given power:


Math.random() generates a number between 0 and 1 randomly:


We can generate any random number by using Math.floor and Math.random together:

Math.floor(Math.random() * 5) + 1;

5 is the max number and 1 is the min.

Global Functions

We can use the String function to convert non-string values to strings:


We can also call toString on primitive values and objects to do the same:


The Number function lets us convert non-numbers to numbers:


decodeURI unescapes URLs:


encodeURI encodes URLs:


We can decode URI components with decodeURIComponent:


And we can encode a string into a URI string with encodeURIComponent :


isFinite lets us check whether a number is finite.

isNaN lets us check whether a value is NaN .

parseFloat lets us parse a value into a floating-point number.

parseInt lets us parse non-number values to integers.


JavaScript regex has the following modifiers:

  • i — perform case-insensitive matching
  • g — perform a global match
  • m — perform multiline matching

And they can have the following patterns:

  • “ — Escape character
  • d — find a digit
  • s — find a whitespace character
  • b — find a match at the beginning or end of a word
  • n+ — contains at least one n
  • n* — contains zero or more occurrences of n
  • n? — contains zero or one occurrence of n
  • ^ — start of string
  • $ — end of string
  • uxxxx — find the Unicode character
  • . — Any single character
  • (a|b) — a or b
  • (...) — Group section
  • [abc] — In range (a, b or c)
  • [0–9] — any of the digits between the brackets
  • [^abc] — Not in range
  • s — White space
  • a? — Zero or one of a
  • a* — Zero or more of a
  • a*? — Zero or more, ungreedy
  • a+ — One or more of a
  • a+? — One or more, ungreedy
  • a{2} — Exactly 2 of a
  • a{2,} — 2 or more of a
  • a{,5} — Up to 5 of a
  • a{2,5} — 2 to 5 of a
  • a{2,5}? — 2 to 5 of a, ungreedy
  • [:punct:] — Any punctu­ation symbol
  • [:space:] — Any space character
  • [:blank:] — Space or tab


JavaScript comes with many useful functions.

We can use regex to match patterns in strings.

JavaScript Basics

JavaScript Cheat Sheet — Errors and Strings

JavaScript is one of the most popular programming languages for web programming.

In this article, we’ll look at the basic syntax of modern JavaScript.


We can use the try-catch block to catch errors from code that may raise errors:

try {
} catch (err) {

We can run code regardless of whether an error is thrown with the finally block:

try {
} catch (err) {
} finally {

We can throw errors by writing:

throw new Error('error')

JavaScript comes with various kinds of error classes:

  • RangeError — A number is out of range
  • ReferenceError — An illegal reference has occurred
  • SyntaxError — A syntax error has occurred
  • TypeError — A type error has occurred
  • URIError — An encodeURI() error has occurred

Input Values

We can get the entered value from an input element with the value property:

const val = document.querySelector("input").value;


We can check for NaN values with isNaN :


Run Code After a Delay

We can run code after a delay with the setTimeout function:

setTimeout(() => {

}, 1000);


We can declare functions with the function keyword:

function addNumbers(a, b) {
  return a + b;;

Update DOM Element Content

We can update DOM element content by setting the innerHTML property:

document.getElementById("elementID").innerHTML = "Hello World";

Output Data

To log data to the console, we call console.log :


We can also show an alert box with alert :


Also, we can show a confirm dialog box by calling confirm :

confirm("Are you sure?");

We can ask the user for inputs with the prompt function:

prompt("What's your age?", "0");


We can add comments to our JavaScript code with // :

// One line

And we can add a multiline comment with:

/* Multi line
comment */


We can declare strings with quotes:

let abc = "abcde";

Also, we can add a new line character with n :

let esc = 'I don't n know';

We get the length of a string with the length property:

let len = abc.length;

We get the index of a substring in a given string with indexOf :


Also, we can get the last occurrence of a substring in a string with lastIndexOf :


We can get a substring between the given indexes with the slice method:

abc.slice(3, 6);

The replace method lets us replace a substring with another substring:


We can convert a string to upper case with toUpperCase :


We can convert a string to upper case with toLowerCase :


We can combine one string with another with concat :

abc.concat(" ", str2);

And we can get the character at the given index with charAt or [] :


The charCodeAt method lets us get the character code at the given index:


The split method lets us split a string by the given separator:


We can split a string by an empty string:


to split a string into an array of characters.

And we can convert a number to a string with the given base with toString :



We can throw and catch errors with JavaScript.

And we can use various methods to work with strings.

JavaScript Basics

JavaScript Cheat Sheet — Operators and Dates

JavaScript is one of the most popular programming languages for web programming.

In this article, we’ll look at the basic syntax of modern JavaScript.


We can do arithmetic with JavaScript.

We add with + and subtract with - :

let a = b + c - d;

We multiply with * and divide with / :

let a = b * (c / d);

And we get the remainder of one number divided by another with / :

let x = 100 % 48;

We increment with ++ :


And we decrement with -- :



We can get type of primitive values and objects with typeof :

typeof a

It’s mostly useful for primitive values since it returns 'object' for all objects.


We assign one value to a variable with = :

let x = 10

The right expression is assigned to the variable on the left.

a +=b is short for a = a + b . We can also replace + with - , * and / .


We compare equality with === :

a === b

We check for inequality with !== :

a !== b

We check if a is greater than b with:

a > b

And we check if a is less than b with:

a < b

We can check for less than or equal with:

a <= b

And we check for greater than or equal with:

a >= b

Logical AND is && :

a && b

And logical OR is || :

a || b


We can create date objects with the Date constructor:

let d = new Date("2017-06-23");

If we omit the month and day, then it’s set to January 1:

let d = new Date("2017");

We can add the hour, minutes, and seconds with:

let d = new Date("2017-06-23T12:00:00-09:45");

Human readable date also works:

let d1 = new Date("June 23 2017");
let d2 = new Date("Jun 23 2017 07:45:00 GMT+0100 (Tokyo Time)");

A date object comes with methods to let us get various values from it.

We call them by writing:

let d = new Date();
let a = d.getDay();

getDay() gets the day of the week

getDate() gets the day of the month as a number.

getFullYear() gets the 4 digit year.

getHours() gets the hours.

getMilliseconds() gets the milliseconds.

getMinutes() gets the minutes.

getMonth() gets the month.

getSeconds() gets the seconds.

getTime() gets the milliseconds since 1970, January 1.

We can also set values with some setter methods.

To call them, we write:

let d = new Date();
d.setDate(d.getDate() + 7);

setDay() sets the day of the week

setDate() sets the day of the month as a number.

setFullYear() sets the 4 digit year.

setHours() sets the hours.

setMilliseconds() sets the milliseconds.

setMinutes() sets the minutes.

setMonth() sets the month.

setSeconds() sets the seconds.

setTime() sets the timestamp.


JavaScript comes with various operators and the Date constructor to let us create, get, and set dates.