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.

Return value

A new string containing the extracted section of the given string. If length is 0 or a negative number, an empty string is returned.

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 compatibility 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

Browser compatibility

See also

• String.prototype.slice()
• String.prototype.substring()