Gets the substring before the specified target.

String.prototype.before.js

/**
 * @license Copyright 2015 - Chris West - MIT Licensed
 * Gets the substring before the specified target.
 * @param {string|RegExp} target
 *     Specifies the substring to key off in order to pull the substring that
 *     precedes it.
 * @param {number=} opt_occurrence
 *     Optional.  The number of the occurrence to key off of.  Defaults to 1.
 * @return {string=}
 *     If the target is found, the substring that precedes will be returned.
 *     Otherwise undefined is returned.
 */
String.prototype.before = function(target, opt_occurrence) {
  var isRegExp = {}.toString.call(target) == '[object RegExp]',
    splits = isRegExp ? [] : this.split(target);
  if (isRegExp) {
    target = target.global
      ? target
      : new RegExp(target.source, 'g' + (target.ignoreCase ? 'i' : '') + (target.multiline ? 'm' : ''));
    this.replace(target, function() {
      splits.push(arguments[arguments.length - 2]);
    });
  }
  return Math.abs(opt_occurrence = opt_occurrence || 1) > splits.length - !isRegExp
    ? null
    : isRegExp
      ? this.slice(0, splits.slice(opt_occurrence - (opt_occurrence > 0))[0])
      : splits.slice(0, opt_occurrence).join(target);
};

Here are some examples:

console.group('Test String.prototype.before()');
console.group('String tests')
console.log('"asdfasdf".before("d", -3) === null:', "asdfasdf".before("d", -3) === null);
console.log('"asdfasdf".before("d", -2) === "as":', "asdfasdf".before("d", -2) === "as");
console.log('"asdfasdf".before("d", -1) === "asdfas":', "asdfasdf".before("d", -1) === "asdfas");
console.log('"asdfasdf".before("d") === "as":', "asdfasdf".before("d") === "as");
console.log('"asdfasdf".before("d", 1) === "as":', "asdfasdf".before("d", 1) === "as");
console.log('"asdfasdf".before("d", 2) === "asdfas":', "asdfasdf".before("d", 2) === "asdfas");
console.log('"asdfasdf".before("d", 3) === null:', "asdfasdf".before("d", 3) === null);
console.groupEnd();
console.group('RegExp tests');
console.log('"asdf".before(/[sD]/i, -3) === null:', "asdf".before(/[sD]/i, -3) === null);
console.log('"asdf".before(/[sD]/i, -2) === "a":', "asdf".before(/[sD]/i, -2) === "a");
console.log('"asdf".before(/[sD]/i, -1) === "as":', "asdf".before(/[sD]/i, -1) === "as");
console.log('"asdf".before(/[sD]/i) === "a":', "asdf".before(/[sD]/i) === "a");
console.log('"asdf".before(/[sD]/i, 1) === "a":', "asdf".before(/[sD]/i, 1) === "a");
console.log('"asdf".before(/[sD]/i, 2) === "as":', "asdf".before(/[sD]/i, 2) === "as");
console.log('"asdf".before(/[sD]/i, 3) === null:', "asdf".before(/[sD]/i, 3) === null);
console.groupEnd();
console.groupEnd();