JavaScript Data Types
String, Number, Boolean, Array, Object, Null, Undefined.
JavaScript Has Dynamic Types
JavaScript has dynamic types. This means that the same variable can be used as different types:
Example
var x = 5; // Now x is a Number
var x = "John"; // Now x is a String
JavaScript Strings
A string is a variable which stores a series of characters like "John Doe".
Strings are written with quotes. You can use single or double quotes:
Example
var carName = 'Volvo XC60'; // Using single quotes
You can use quotes inside a string, as long as they don't match the quotes surrounding the string:
Example
var answer = "He is called 'Johnny'"; // Single quotes inside double quotes
var answer = 'He is called "Johnny"'; // Double quotes inside single quotes
Try it yourself »
You will learn a lot more about strings later in this tutorial.
JavaScript Numbers
JavaScript has only one type of numbers.
Numbers can be written with, or without decimals:
Example
var x2 = 34; // Written without decimals
Extra large or extra small numbers can be written with scientific (exponential) notation:
You will learn a lot more about numbers in the advanced section of this tutorial.
JavaScript Booleans
Booleans can only have two values: true or false.
var y = false;
Booleans are often used in conditional testing.
You will learn a lot 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):
Array indexes are zero-based, which means the first item is [0], second is [1], and so on.
You will learn a lot 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.
The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.
You will learn a lot more about objects later in this tutorial.
Undefined and Null
The value of a variable with no value is undefined.
Variables can be emptied by setting the value to null.
The typeof Operator
You can use the JavaScript typeof operator to find the type of a variable.
Example
typeof 3.14 // Returns number
typeof false // Returns boolean
typeof [1,2,3,4] // Returns object
typeof {name:'John', age:34} // Returns object
Try it yourself »
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In JavaScript, an array is a special type of object. Therefore typeof [1,2,3,4] returns object. |
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Do Not Declare String, Number, and Boolean as Objects!
When a JavaScript variable is declared with the keyword "new", the variable is created as an object:
var y = new Number(); // Declares y as a Number object
var z = new Boolean(); // Declares z as a Boolean object
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Avoid String, Number, and Boolean objects. They complicates your code and slow down execution speed. |
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