function weightedMean(arrValues, arrWeights) {

  var result = arrValues.map(function (value, i) {

    var weight = arrWeights[i];
    var sum = value * weight;

    return [sum, weight];
  }).reduce(function (p, c) {

    return [p[0] + c[0], p[1] + c[1]];
  }, [0, 0]);

  return result[0] / result[1];
}

weightedMean([251, 360, 210], [0.1, 0.5, 0.7]);
// => 270.8461538461539