JavaScript prototype Property: String Object


The prototype property is used to add new properties and methods to an object.


Implemented in JavaScript 1.1


myobj.prototype.name = value 

myobj: The name of the constructor function object you want to change

name: The name of the property or method to be created.

value: The value initially assigned to the new property or method.

If you add a property to the prototype for an object, then all objects created with that object's constructor function will have that new property, even if the objects existed before you created the new property.


The following web document demonstrates how the prototype property can be used.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<meta http-equiv="content-type" content="text/html; charset=iso-8859-1" />
<title>JavaScript String object - prototype property example</title>
<script type="text/javascript">
//This is done to make the following JavaScript code compatible to XHTML. <![CDATA[
function stringgreen()
return "<font color=green ><b>  " +this.toString() + "</b></font>";
<h1 style="color: red">JavaScript Number object : prototype property</h1>
<hr />
<script type="text/javascript">
//This is done to make the following JavaScript code compatible to XHTML. <![CDATA[
String.prototype.string_green = stringgreen

View the example in the browser

Supported Browser

Internet Explorer 7 Firefox 3.6 Google Chrome 7 Safari 5.0.1 Opera 10
Yes Yes Yes Yes Yes

See also:

JavaScript Core objects, methods, properties.

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JavaScript: Tips of the Day

How to check whether a string contains a substring in JavaScript?

ECMAScript 6 introduced String.prototype.includes:

const string = "foo";
const substring = "oo";


includes doesn't have Internet Explorer support, though. In ECMAScript 5 or older environments, use String.prototype.indexOf, which returns -1 when a substring cannot be found:

var string = "foo";
var substring = "oo";

console.log(string.indexOf(substring) !== -1);

Ref: https://bit.ly/3fFFgZv