手写Array.prototype.reduce

May 21, 2020

前端知识复习 code snippet

Let O be ? ToObject(this value).

Let len be ? LengthOfArrayLike(O).

If IsCallable(callbackfn) is false, throw a TypeError exception.

If len is 0 and initialValue is not present, throw a TypeError exception.

Let k be 0.

Let accumulator be undefined.

If initialValue is present, then

Set accumulator to initialValue.

Else,

Let kPresent be false.

Repeat, while kPresent is false and k < len,

Let Pk be ! ToString(k).

Set kPresent to ? HasProperty(O, Pk).

If kPresent is true, then

Set accumulator to ? Get(O, Pk).

Set k to k + 1.

If kPresent is false, throw a TypeError exception.

Repeat, while k < len,

Let Pk be ! ToString(k).

Let kPresent be ? HasProperty(O, Pk).

If kPresent is true, then

Let kValue be ? Get(O, Pk).

Set accumulator to ? Call(callbackfn, undefined, « accumulator, kValue, k, O »).

Set k to k + 1.

Return accumulator.

代码

Array.prototype.myReduce = function(callbackfn, initialValue) {
  // this must not be null or undefined
  if (this == null) {
    throw new TypeError('Array.prototype.reduce called on null or undefined');
  }
  // callbackfn must be a function
  if (typeof callbackfn !== 'function') {
    throw new TypeError(callbackfn + ' is not a function');
  }

  var O = Object(this);
  var len = O.length >>> 0;

  var k = 0,
    accumulator = initialValue;
  // get initialValue
  find_initial: if (accumulator == undefined) {
    for (; k < len; k++) {
      if (k in O) {
        accumulator = O[k++];
        break find_initial;
      }
    }
    // end of iteration with no initialValue found, throw empty array error
    throw new TypeError('myReduce of empty array with no initial value');
  }

  // iterate array and merge results
  for (; k < len; k++) {
    if (k in O) {
      var kValue = O[k];
      accumulator = callbackfn(accumulator, kValue, k, O);
    }
  }
  return accumulator;
};