Warning: Although String.prototype.substr(…) is not strictly deprecated (as in "removed from the Web standards"), it is considered a legacy function and should be avoided when possible. It is not part of the core JavaScript language and may be removed in the future. If at all possible, use the substring() method instead.

The substr() method returns a portion of the string, starting at the specified index and extending for a given number of characters afterward.

Syntax

str.substr(start[, length])

Parameters

start
The index of the first character to include in the returned substring.
length
Optional. The number of characters to extract.

Return value

A new string containing the specified part of the given string.

Description

substr() extracts length characters from a string, counting from the start index.

If start is a positive number, the index starts counting at the start of the string. Its value is capped at str.length.
If start is a negative number, the index starts counting from the end of the string. Its value is capped at -str.length.
Note: In Microsoft JScript, negative values of the start argument are not considered to refer to the end of the string.

If length is omitted, substr() extracts characters to the end of the string.
If length is undefined, substr() extracts characters to the end of the string.
If length is a negative number, it is treated as 0.

For both start and length, NaN is treated as 0.

Examples

Using substr()

var aString = 'Mozilla';

console.log(aString.substr(0, 1));   // 'M'
console.log(aString.substr(1, 0));   // ''
console.log(aString.substr(-1, 1));  // 'a'
console.log(aString.substr(1, -1));  // ''
console.log(aString.substr(-3));     // 'lla'
console.log(aString.substr(1));      // 'ozilla'
console.log(aString.substr(-20, 2)); // 'Mo'
console.log(aString.substr(20, 2));  // ''

Polyfill

Microsoft's JScript does not support negative values for the start index. To use this feature in JScript, you can use the following code:

// only run when the substr() function is broken
if ('ab'.substr(-1) != 'b') {
  /**
   *  Get the substring of a string
   *  @param  {integer}  start   where to start the substring
   *  @param  {integer}  length  how many characters to return
   *  @return {string}
   */
  String.prototype.substr = function(substr) {
    return function(start, length) {
      // call the original method
      return substr.call(this,
      	// did we get a negative start, calculate how much it is from the beginning of the string
        // adjust the start parameter for negative value
        start < 0 ? this.length + start : start,
        length)
    }
  }(String.prototype.substr);
}

Specifications

Specification Status Comment
ECMAScript 3rd Edition (ECMA-262) Standard Defined in the (informative) Compatibility Annex B. Implemented in JavaScript 1.0.
ECMAScript 5.1 (ECMA-262)
The definition of 'String.prototype.substr' in that specification.
Standard Defined in the (informative) Compatibility Annex B
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'String.prototype.substr' in that specification.
Standard Defined in the (normative) Annex B for Additional ECMAScript Features for Web Browsers
ECMAScript Latest Draft (ECMA-262)
The definition of 'String.prototype.substr' in that specification.
Draft Defined in the (normative) Annex B for Additional ECMAScript Features for Web Browsers

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic support
Deprecated
Chrome Full support YesEdge Full support YesFirefox Full support 1IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes

Legend

Full support  
Full support
Deprecated. Not for use in new websites.
Deprecated. Not for use in new websites.

See also