The substr() method returns the part of a string between the start index and a number of characters after it.

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

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes Yes1 Yes Yes Yes
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes4 Yes Yes Yes

See also