String.prototype.substr()

Summary

The substr() method returns the characters in a string beginning at the specified location through the specified number of characters.

Syntax

str.substr(start[, length])

Parameters

start
Location at which to begin extracting characters.
length
The number of characters to extract.

Description

start is a character index. The index of the first character is 0, and the index of the last character is 1 less than the length of the string. substr begins extracting characters at start and collects length characters (unless it reaches the end of the string first, in which case it will return fewer).

If start is positive and is greater than or equal to the length of the string, substr returns an empty string.

If start is negative, substr uses it as a character index from the end of the string. If start is negative and abs(start) is larger than the length of the string, substr uses 0 as the start index. Note: the described handling of negative values of the start argument is not supported by Microsoft JScript .

If length is 0 or negative, substr returns an empty string. If length is omitted, substr extracts characters to the end of the string.

Examples

Example: Using substr

var str = "abcdefghij";

console.log("(1,2): "    + str.substr(1,2));   // (1,2): bc
console.log("(-3,2): "   + str.substr(-3,2));  // (-3,2): hi
console.log("(-3): "     + str.substr(-3));    // (-3): hij
console.log("(1): "      + str.substr(1));     // (1): bcdefghij
console.log("(-20, 2): " + str.substr(-20,2)); // (-20, 2): ab
console.log("(20, 2): "  + str.substr(20,2));  // (20, 2):

Polyfill

Microsoft's JScript does not support negative values for the start index. If you wish to make use of this feature, you can use the following compatibilty code to work around this bug:

// 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) {
      // did we get a negative start, calculate how much it is
      // from the beginning of the string
      if (start < 0) start = this.length + start;
      
      // call the original function
      return substr.call(this, start, length);
    }
  }(String.prototype.substr);
}

Specifications

Specification Status Comment
ECMAScript 3rd Edition. Standard Defined in the (informative) Compatibility Annex B.
Implemented in JavaScript 1.0
ECMAScript Language Specification 5.1th Edition (ECMA-262) Standard Defined in the (informative) Compatibility Annex B
ECMAScript Language Specification 6th Edition (ECMA-262) Draft Defined in the (normative) Annex B for Additional ECMAScript Features for Web Browsers

Browser compatibility

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

See also

Document Tags and Contributors

Last updated by: fscholz,
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