Creating the underscore reduce function from scratch

I am working on creating my own callback functions and higher order function groups. I am stuck on replicating the underscore reduce function or ._reduce function. Can someone help me understand how it works underneath the hood it has been a few days for me and I am stumped. Here is what I have so far. Please understand I am not utilizing the underscore library, I am trying to replicate it so that I can further my understanding on higher order functions. Thanks.

var reduce = function(collection, iterator, accumulator) {

    var iterator = function(startPoint, combiner){
      for(var i = 0; i <combiner.length; i++){
        startPoint += combiner[i];
    }
    return iterator(accumulator, collection);
}

---

In the comments of these answers, there has been a lot of confusion between Underscore's reduce and Array.prototype.reduce . Two notes:

Underscore's reduce allows for an empty collection and no seed value. In this case, it will not throw an error but rather returns undefined . naomik has convinced me that this is not safe. For example _([]).reduce(function(a, b) { return a + b}); should either throw an error or return an empty list.

Underscore's reduce works on both objects and arrays.

Now, to my original post:

---

I actually did the same thing—implement the key functions from Underscore from scratch—a while back, and reduce was probably the trickiest. I think reduce is easier to grok with a non-functional reduce (credit to naomik for this):

function reduce(arr, func, seed) {
    var result = seed,
        len = arr.length,
        i = 0;
    for (; i < len; i++) {
        result = func(result, arr[i])
     }
     return result
 }

Underscore's implementation is a bit more complex, handling objects and arrays, empty collections, and an optional seed value. It also uses each instead of a for loop, since that is more functional in style. This is my implementation of Underscore's reduce :

var reduce = function(coll, func, seed) {
    // `isEmpty` (not shown) handles empty arrays, strings, and objects.
    // Underscore accepts an optional seed value and does not 
    // throw an error if given an empty collection and no seed.
    if (isEmpty(coll)) {
        return coll;
    }
    var noSeed = arguments.length < 3;

    // `each` (not shown) should treat arrays and objects
    // in the same way.
    each(coll, function(item, i) {
        if (noSeed) {
            // This condition passes at most once. If it passes,
            // this means the user did not provide a seed value.
            // Default to the first item in the list.
            noSeed = false;
            seed = item;
        } else {
            seed = func(seed, item, i);
        }
    });

    return seed;
};

---

一个简单的递归函数可以做到这一点

// arr - some array of values
// f   - the reducing function
// acc - initial value for the accumulator
function reduce(arr, f, acc) {
  if (arr.length === 0)
    return acc
  else
    return reduce(arr.slice(1), f, f(acc, arr[0]))
}

// --------------------------------------------------
   
// example 1:
// reduce an array of numbers using an adding function

var ex1 = reduce([1,2,3], function(acc, x) { return acc + x }, 0)

console.log(ex1)
//=> 6

// --------------------------------------------------

// example 2:
// reduce an array of pairs to a mapping object

var ex2 = reduce([['a', 1], ['b', 2], ['c', 3]], function(acc, pair) {
  var key = pair[0]
  var value = pair[1]
  acc[key] = value
  return acc
}, {})

console.log(ex2)
//=> { a: 1, b: 2, c: 3 }

---

It's something like it:

function reduce(array, combine, start) {
  for (var i = 0; i < array.length; i++)
    start = combine(start, array[i]);
  return start;
}

console.log(reduce([1, 2, 3, 4], function(a, b) {
  return a + b;
}, 0));

Link Reference: http://eloquentjavascript.net/05_higher_order.html

链接地址: http://www.djcxy.com/p/34968.html

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