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JS Objects

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JS Classes

Class Intro Class Inheritance Class Static

JS Async

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DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM Forms DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

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JS References

JavaScript Objects HTML DOM Objects

JavaScript Data Types

JavaScript has 8 Datatypes

1. String
2. Number
3. Bigint
4. Boolean
5. Undefined
6. Null
7. Symbol
8. Object

The Object Datatype

The object data type can contain:

1. An object
2. An array
3. A date


// Numbers:
let length = 16;
let weight = 7.5;

// Strings:
let color = "Yellow";
let lastName = "Johnson";

// Booleans
let x = true;
let y = false;

// Object:
const person = {firstName:"John", lastName:"Doe"};

// Array object:
const cars = ["Saab", "Volvo", "BMW"];

// Date object:
const date = new Date("2022-03-25");


A JavaScript variable can hold any type of data.

The Concept of Data Types

In programming, data types is an important concept.

To be able to operate on variables, it is important to know something about the type.

Without data types, a computer cannot safely solve this:

let x = 16 + "Volvo";

Does it make any sense to add "Volvo" to sixteen? Will it produce an error or will it produce a result?

JavaScript will treat the example above as:

let x = "16" + "Volvo";


When adding a number and a string, JavaScript will treat the number as a string.


let x = 16 + "Volvo";
Try it Yourself »


let x = "Volvo" + 16;
Try it Yourself »

JavaScript evaluates expressions from left to right. Different sequences can produce different results:


let x = 16 + 4 + "Volvo";


Try it Yourself »


let x = "Volvo" + 16 + 4;


Try it Yourself »

In the first example, JavaScript treats 16 and 4 as numbers, until it reaches "Volvo".

In the second example, since the first operand is a string, all operands are treated as strings.

JavaScript Types are Dynamic

JavaScript has dynamic types. This means that the same variable can be used to hold different data types:


let x;       // Now x is undefined
x = 5;       // Now x is a Number
x = "John";  // Now x is a String
Try it Yourself »

JavaScript Strings

A string (or a text string) is a series of characters like "John Doe".

Strings are written with quotes. You can use single or double quotes:


// Using double quotes:
let carName1 = "Volvo XC60";

// Using single quotes:
let carName2 = 'Volvo XC60';
Try it Yourself »

You can use quotes inside a string, as long as they don't match the quotes surrounding the string:


// Single quote inside double quotes:
let answer1 = "It's alright";

// Single quotes inside double quotes:
let answer2 = "He is called 'Johnny'";

// Double quotes inside single quotes:
let answer3 = 'He is called "Johnny"';
Try it Yourself »

You will learn more about strings later in this tutorial.

JavaScript Numbers

All JavaScript numbers are stored as decimal numbers (floating point).

Numbers can be written with, or without decimals:


// With decimals:
let x1 = 34.00;

// Without decimals:
let x2 = 34;
Try it Yourself »

Exponential Notation

Extra large or extra small numbers can be written with scientific (exponential) notation:


let y = 123e5;    // 12300000
let z = 123e-5;   // 0.00123
Try it Yourself »


Most programming languages have many number types:

Whole numbers (integers):
byte (8-bit), short (16-bit), int (32-bit), long (64-bit)

Real numbers (floating-point):
float (32-bit), double (64-bit).

Javascript numbers are always one type:
double (64-bit floating point).

You will learn more about numbers later in this tutorial.

JavaScript BigInt

All JavaScript numbers are stored in a a 64-bit floating-point format.

JavaScript BigInt is a new datatype (ES2020) that can be used to store integer values that are too big to be represented by a normal JavaScript Number.


let x = BigInt("123456789012345678901234567890");
Try it Yourself »

You will learn more about BigInt later in this tutorial.

JavaScript Booleans

Booleans can only have two values: true or false.


let x = 5;
let y = 5;
let z = 6;
(x == y)       // Returns true
(x == z)       // Returns false
Try it Yourself »

Booleans are often used in conditional testing.

You will learn more about booleans later in this tutorial.

JavaScript Arrays

JavaScript arrays are written with square brackets.

Array items are separated by commas.

The following code declares (creates) an array called cars, containing three items (car names):


const cars = ["Saab", "Volvo", "BMW"];
Try it Yourself »

Array indexes are zero-based, which means the first item is [0], second is [1], and so on.

You will learn more about arrays later in this tutorial.

JavaScript Objects

JavaScript objects are written with curly braces {}.

Object properties are written as name:value pairs, separated by commas.


const person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
Try it Yourself »

The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.

You will learn more about objects later in this tutorial.

The typeof Operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable.

The typeof operator returns the type of a variable or an expression:


typeof ""             // Returns "string"
typeof "John"         // Returns "string"
typeof "John Doe"     // Returns "string"
Try it Yourself »


typeof 0              // Returns "number"
typeof 314            // Returns "number"
typeof 3.14           // Returns "number"
typeof (3)            // Returns "number"
typeof (3 + 4)        // Returns "number"
Try it Yourself »

You will learn more about typeof later in this tutorial.


In JavaScript, a variable without a value, has the value undefined. The type is also undefined.


let car;    // Value is undefined, type is undefined
Try it Yourself »

Any variable can be emptied, by setting the value to undefined. The type will also be undefined.


car = undefined;    // Value is undefined, type is undefined
Try it Yourself »

Empty Values

An empty value has nothing to do with undefined.

An empty string has both a legal value and a type.


let car = "";    // The value is "", the typeof is "string"
Try it Yourself »

Test Yourself With Exercises


Use comments to describe the correct data type of the following variables:

let length = 16;            // 
let lastName = "Johnson";   // 
const x = {
  firstName: "John",
  lastName: "Doe"
};                          // 

Start the Exercise