Learn JS #8

JavaScript Data Types

JavaScript variables can hold different data types: numbers, strings, objects and more:

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

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";


let x = "Volvo" + 16;

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


let x = 16 + 4 + "Volvo";




let x = "Volvo" + 16 + 4;



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

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:


let carName1 = "Volvo XC60";   // Using double quotes
let carName2 = 'Volvo XC60';   // Using single quotes

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


let answer1 = "It's alright";             // Single quote inside double quotes
let answer2 = "He is called 'Johnny'";    // Single quotes inside double quotes
let answer3 = 'He is called "Johnny"';    // Double quotes inside single quotes

You will learn more about strings later in this tutorial.

JavaScript Numbers

JavaScript has only one type of numbers.

Numbers can be written with, or without decimals:


let x1 = 34.00;     // Written with decimals
let x2 = 34;        // Written without decimals

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


let y = 123e5;      // 12300000
let z = 123e-5;     // 0.00123

You will learn more about numbers 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

Booleans are often used in conditional testing.

You will learn more about conditional testing 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"];

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"};

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"


typeof 0              // Returns "number"
typeof 314            // Returns "number"
typeof 3.14           // Returns "number"
typeof (3)            // Returns "number"
typeof (3 + 4)        // Returns "number"

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

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

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"


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