string.substr

str.substr(start, [length])

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): '

// 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
}