Parse query string, accepting multiple parameters

getQueryParams.js

/**
 * Parse query string.
 * ?a=b&c=d&e=f&e=g to {a: 'b', c: 'd', e: ['f', 'g']}
 * @param {String} (optional) queryString
 * @return {Object} query params
 */
getQueryParams: function(queryString) {
  var query = (queryString || window.location.search).replace(/^[?]/, ''),
      result = {};
  if (!query) return result;
  var pieces = query.split('&');
  for (var i = 0, s = pieces[i]; i < pieces.length; s = pieces[++i]) {
    var pair = s.split('='),
        key = pair[0],
        val = pair[1] && decodeURIComponent(pair[1]);
    if (key in result) {
      var current = result[key];
      if (current instanceof Array) {
        current.push(val)
      } else {
        result[key] = [current, val];
      }
    } else {
      result[key] = val;
    }
  }
  return result;
}
 
// --- Jasmine test
 
describe('_.getQueryParams', function() {
  it('?a=1', function() {
    var result = _.getQueryParams('?a=1');
    expect(result).toEqual({a: '1'});
  });
  it('?a=1&b=2', function() {
    var result = _.getQueryParams('?a=1&b=2');
    expect(result).toEqual({a: '1', b: '2'});
  });
  it('?a=1&jp=ほげ&en=Hoge', function() {
    var result = _.getQueryParams('?a=1&jp=%E3%81%BB%E3%81%92&en=Hoge');
    expect(result).toEqual({a: '1', jp: 'ほげ', en: 'Hoge'});
  });
  it('?a=1&jp=%E3%81%BB%E3%81%92&en=Hoge', function() {
    var result = _.getQueryParams('?a=1&jp=ほげ&en=Hoge');
    expect(result).toEqual({a: '1', jp: 'ほげ', en: 'Hoge'});
  });
  it('?a&b=2', function() {
    var result = _.getQueryParams('?a&b=2');
    expect(result).toEqual({a: 'undefined', b: '2'});
  });
  it('', function() {
    var result = _.getQueryParams('');
    expect(result).toEqual({});
  });
  it('?n=x&n=y', function() {
    var result = _.getQueryParams('?n=x&n=y');
    expect(result).toEqual({n: ['x', 'y']});
  });
});