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    String.prototype.substr()

    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. If a negative number is given, it is treated as strLength + start where strLength is the length of the string (for example, if start is -3 it is treated as strLength - 3.)
    length
    Optional. 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

    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) {
          // 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. 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

    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)

    Note: Up to version 3.6, Firefox had a bug which caused substr() to return empty result when an explicit undefined value was passed in as the length.

    See also

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