JavaScript 使用 reduce 方法实现 forEach、map、filter

// forEach
function forEachUseReduce(array, handler) {
  array.reduce(function (pre, item, index) {
    handler(item, index);
  });
}
// map
function mapUseReduce(array, handler) {
  let result = [];
  array.reduce(function (pre, item, index) {
    let mapItem = handler(item, index);
    result.push(mapItem);
  });
  return result;
}
// filter
function filterUseReduce(array, handler) {
  let result = [];
  array.reduce(function (pre, item, index) {
    if (handler(item, index)) {
      result.push(item);
    }
  });
  return result;
}