1.5 Data Types

1.5 Data Types

1.5 Data Types

JavaScript has different data types: numbers, booleans, strings, objects, and arrays, and more.
Here are some examples:
let length = 16; // type: number let lastName = "Johnson"; // type: string let obj = { firstName: "John", lastName: "Doe" }; // type: object

What exactly are data types?

In programming, data types are an important concept. It tells the computer what a piece of code is and how it should be treated.
Let's go over the different types:


Every number you can think of fall under the number type.
Numbers can be written with or without decimals.
let numOne = 34.00; // written with decimals let numTwo = 34; // written without decimals
Extra large or extra small numbers can be written with scientific notation.
let x = 123e5; // 12300000 let y = 123e-5; // 0.00123


Booleans can only have two values: true or false.
let isRaining = false; let isSunny = true;


A string (or a text string) is a series of characters like this sentence.
Strings are written with quotes. You can use single or double quotes.
let carNameOne = "Volvo XC60"; // using double quotes let carNameTwo = 'Wolwo YT90'; // 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


Arrays are written with square brackets. The items are separated by commas.
The following code declares (creates) an array called sports, containing three items (sport names):
let sports = ["Basketball", "Soccer", "Badminton"];


Objects are written with curly braces.
The object properties are written as name:value. The properties are separated by commas.
// note: this is one line let person = { firstName: "John", lastName: "Doe", age: 50, eyeColor: "Blue" };
The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.

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 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"

Empty values

An empty value has nothing to with undefined.
An empty string has both a legal value and type.
let car = ""; // the value is "", the type is string


A variable without a value has the value undefined. The type is also known as 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.
let car = "Volvo XP90"; car = undefined; // value is undefined, type is undefined


null is "nothing". It is supposed to be something that doesn't exist. However, the data type of null is considered a object.
You can consider it a buy in JavaScript that typeof null is an object. It should be null.
You can empty an object by setting it to null.
let person = { firstName: "John", lastName: "Doe", age: 50, eyeColor: "blue" }; person = null; // now value is null, but type is still an object
You can also empty an object by setting it to undefined.
let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"}; person = undefined; // now both value and type is undefined

Difference between undefined and null

undefined and null are equal in value, but different in type.
typeof undefined; // undefined typeof null; // object

Complex Data

The typeof operator can return one of two of complex types: function and object.
The typeof operator returns object for objects, arrays, and null.
The typeof operator does not return object for functions.
typeof { name:'John', age: 34 }; // returns "object" typeof [1,2,3,4]; // returns "object" (not "array", see note below) typeof null; // returns "object" typeof function myFunc(){}; // returns "function"
The typeof operator returns object for arrays are objects.
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