Revision 507401 of String.prototype.indexOf()

  • Revision slug: Web/JavaScript/Reference/Global_Objects/String/indexOf
  • Revision title: String.prototype.indexOf()
  • Revision id: 507401
  • Created:
  • Creator: fscholz
  • Is current revision? No
  • Comment cleanup

Revision Content

{{JSRef("Global_Objects", "String")}}

Summary

The indexOf() method returns the index within the calling String object of the first occurrence of the specified value, starting the search at fromIndex,
returns -1 if the value is not found.

Syntax

str.indexOf(searchValue[, fromIndex])

Parameters

searchValue
A string representing the value to search for.
fromIndex
The location within the calling string to start the search from. It can be any integer between 0 and the length of the string. The default value is 0.

Description

Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character of a string called stringName is stringName.length - 1.

"Blue Whale".indexOf("Blue");     // returns  0
"Blue Whale".indexOf("Blute");    // returns -1
"Blue Whale".indexOf("Whale", 0); // returns  5
"Blue Whale".indexOf("Whale", 5); // returns  5
"Blue Whale".indexOf("", 9);      // returns  9
"Blue Whale".indexOf("", 10);     // returns 10
"Blue Whale".indexOf("", 11);     // returns 10

Case-sensitivity

The indexOf method is case sensitive. For example, the following expression returns -1:

"Blue Whale".indexOf("blue") // returns -1

Checking occurrences

Note that '0' doesn't evaluate to true and '-1' doesn't evaluate to false. Therefore, when checking if a specific string exists within another string the correct way to check would be:

"Blue Whale".indexOf("Blue") != -1; // true
"Blue Whale".indexOf("Bloe") != -1; // false

Examples

Example: Using indexOf and lastIndexOf

The following example uses indexOf and lastIndexOf to locate values in the string "Brave new world".

var anyString = "Brave new world";

console.log( "The index of the first w from the beginning is " + anyString.indexOf("w") );     // Displays 8
console.log( "The index of the first w from the end is "       + anyString.lastIndexOf("w") ); // Displays 10

console.log( "The index of 'new' from the beginning is " + anyString.indexOf("new") );    // Displays 6
console.log( "The index of 'new' from the end is "       + anyString.lastIndexOf("new")); // Displays 6

Example: indexOf and case-sensitivity

The following example defines two string variables. The variables contain the same string except that the second string contains uppercase letters. The first log method displays 19. But because the indexOf method is case sensitive, the string "cheddar" is not found in myCapString, so the second log method displays -1.

var myString    = "brie, pepper jack, cheddar";
var myCapString = "Brie, Pepper Jack, Cheddar";

console.log('myString.indexOf("cheddar") is '    + myString.indexOf("cheddar"));    // Displays 19
console.log('myCapString.indexOf("cheddar") is ' + myCapString.indexOf("cheddar")); // Displays -1

Example: Using indexOf to count occurrences of a letter in a string

The following example sets count to the number of occurrences of the letter x in the string str:

count = 0;
pos = str.indexOf("x");

while ( pos != -1 ) {
   count++;
   pos = str.indexOf( "x",pos + 1 );
}

Specifications

Specification Status Comment
ECMAScript 1st Edition. Standard Initial definition.
{{SpecName('ES5.1', '#sec-15.5.4.7', 'String.prototype.indexOf')}} {{Spec2('ES5.1')}}  
{{SpecName('ES6', '#sec-string.prototype.indexof', 'String.prototype.indexOf')}} {{Spec2('ES6')}}  

Browser compatibility

{{ CompatibilityTable() }}

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }}
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }} {{ CompatVersionUnknown() }}

See also

  • {{jsxref("String.prototype.charAt()")}}
  • {{jsxref("String.prototype.lastIndexOf()")}}
  • {{jsxref("String.prototype.split()")}}
  • {{jsxref("Array.prototype.indexOf()")}

Revision Source

<p>{{JSRef("Global_Objects", "String")}}</p>
<h2 id="Summary" name="Summary">Summary</h2>
<p>The <code><strong>indexOf()</strong></code> method returns the index within the calling <code>String</code> object of the first occurrence of the specified value, starting the search at <code>fromIndex</code>,<br />
 returns -1 if the value is not found.</p>
<h2 id="Syntax" name="Syntax">Syntax</h2>
<pre class="syntaxbox">
<code><em>str</em>.indexOf(<em>searchValue</em>[, <em>fromIndex</em>]</code>)</pre>
<h3 id="Parameters" name="Parameters">Parameters</h3>
<dl>
 <dt>
  <code>searchValue</code></dt>
 <dd>
  A string representing the value to search for.</dd>
 <dt>
  <code>fromIndex</code></dt>
 <dd>
  The location within the calling string to start the search from. It can be any integer between 0 and the length of the string. The default value is 0.</dd>
</dl>
<h2 id="Description" name="Description">Description</h2>
<p>Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character of a string called <code>stringName</code> is <code>stringName.length - 1</code>.</p>
<pre class="brush: js">
"Blue Whale".indexOf("Blue");     // returns  0
"Blue Whale".indexOf("Blute");    // returns -1
"Blue Whale".indexOf("Whale", 0); // returns  5
"Blue Whale".indexOf("Whale", 5); // returns  5
"Blue Whale".indexOf("", 9);      // returns  9
"Blue Whale".indexOf("", 10);     // returns 10
"Blue Whale".indexOf("", 11);     // returns 10</pre>
<h3 id="Example:_indexOf_and_case-sensitivity" name="Example:_indexOf_and_case-sensitivity">Case-sensitivity</h3>
<p>The <code>indexOf</code> method is case sensitive. For example, the following expression returns -1:</p>
<pre class="brush: js">
"Blue Whale".indexOf("blue") // returns -1
</pre>
<h3>Checking occurrences</h3>
<p>Note that '0' doesn't evaluate to <code>true</code> and '-1' doesn't evaluate to <code>false</code>. Therefore, when checking if a specific string exists within another string the correct way to check would be:</p>
<pre class="brush: js">
"Blue Whale".indexOf("Blue") != -1; // true
"Blue Whale".indexOf("Bloe") != -1; // false</pre>
<h2 id="Examples" name="Examples">Examples</h2>
<h3 id="Example:_Using_indexOf_and_lastIndexOf" name="Example:_Using_indexOf_and_lastIndexOf">Example: Using <code>indexOf</code> and <code>lastIndexOf</code></h3>
<p>The following example uses <code>indexOf</code> and <code>lastIndexOf</code> to locate values in the string "<code>Brave new world</code>".</p>
<pre class="brush: js">
var anyString = "Brave new world";

console.log( "The index of the first w from the beginning is " + anyString.indexOf("w") );     // Displays 8
console.log( "The index of the first w from the end is "       + anyString.lastIndexOf("w") ); // Displays 10

console.log( "The index of 'new' from the beginning is " + anyString.indexOf("new") );    // Displays 6
console.log( "The index of 'new' from the end is "       + anyString.lastIndexOf("new")); // Displays 6
</pre>
<h3 id="Example:_indexOf_and_case-sensitivity" name="Example:_indexOf_and_case-sensitivity">Example: <code>indexOf</code> and case-sensitivity</h3>
<p>The following example defines two string variables. The variables contain the same string except that the second string contains uppercase letters. The first <code>log</code> method displays 19. But because the <code>indexOf</code> method is case sensitive, the string "<code>cheddar</code>" is not found in <code>myCapString</code>, so the second <code>log</code> method displays -1.</p>
<pre class="brush: js">
var myString    = "brie, pepper jack, cheddar";
var myCapString = "Brie, Pepper Jack, Cheddar";

console.log('myString.indexOf("cheddar") is '    + myString.indexOf("cheddar"));    // Displays 19
console.log('myCapString.indexOf("cheddar") is ' + myCapString.indexOf("cheddar")); // Displays -1</pre>
<h3 id="Example:_Using_indexOf_to_count_occurrences_of_a_letter_in_a_string" name="Example:_Using_indexOf_to_count_occurrences_of_a_letter_in_a_string">Example: Using <code>indexOf</code> to count occurrences of a letter in a string</h3>
<p>The following example sets <code>count</code> to the number of occurrences of the letter <code>x</code> in the string <code>str</code>:</p>
<pre class="brush: js">
count = 0;
pos = str.indexOf("x");

while ( pos != -1 ) {
   count++;
   pos = str.indexOf( "x",pos + 1 );
}</pre>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>ECMAScript 1st Edition.</td>
   <td>Standard</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-15.5.4.7', 'String.prototype.indexOf')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-string.prototype.indexof', 'String.prototype.indexOf')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>&nbsp;</td>
  </tr>
 </tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>{{ CompatibilityTable() }}</p>
<div id="compat-desktop">
 <table class="compat-table">
  <tbody>
   <tr>
    <th>Feature</th>
    <th>Chrome</th>
    <th>Firefox (Gecko)</th>
    <th>Internet Explorer</th>
    <th>Opera</th>
    <th>Safari</th>
   </tr>
   <tr>
    <td>Basic support</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
   </tr>
  </tbody>
 </table>
</div>
<div id="compat-mobile">
 <table class="compat-table">
  <tbody>
   <tr>
    <th>Feature</th>
    <th>Android</th>
    <th>Chrome for Android</th>
    <th>Firefox Mobile (Gecko)</th>
    <th>IE Mobile</th>
    <th>Opera Mobile</th>
    <th>Safari Mobile</th>
   </tr>
   <tr>
    <td>Basic support</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
   </tr>
  </tbody>
 </table>
</div>
<h2 id="See_also" name="See_also">See also</h2>
<ul>
 <li>{{jsxref("String.prototype.charAt()")}}</li>
 <li>{{jsxref("String.prototype.lastIndexOf()")}}</li>
 <li>{{jsxref("String.prototype.split()")}}</li>
 <li>{{jsxref("Array.prototype.indexOf()")}</li>
</ul>
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