使用 reduce 方法实现 forEach、map、filter ​

// forEach
function forEachUseReduce(array, handler) {
  array.reduce(function (pre, item, index) {
    handler(item, index);
  }, null);
}

// map
function mapUseReduce(array, handler) {
  let result = [];

  array.reduce(function (pre, item, index) {
    let mapItem = handler(item, index);
    result.push(mapItem);
  }, null);

  return result;
}

// filter
function filterUseReduce(array, handler) {
  let result = [];

  array.reduce(function (pre, item, index) {
    if (handler(item, index)) {
      result.push(item);
    }
  }, null);

  return result;
}