js减少只有空值的数组

Question

Why doesn't this iterate?

(Array(5)).reduce(function(cur,prv,n,a){alert(n)},'');

I never reached the function body, seemingly ignoring all of the empty values, which is not what I want it to do.

Answer 1

As far as I understand Array(5) returns an instance of Array Object with a property length (value 5), but without values. One way to be able to use reduce (or other array methods like map ) here is:

String(Array(5)).split(',').reduce(function (cur, prv, n, a) { /* ... */ })

An alternative could be:

Array.apply(null,{length: 5}).map( [callback] );
// or
Array.apply(null,Array(5)).map( [callback] );

But that will encounter a maximum call stack error at some point. The string/split method keeps working (albeit a bit slower) for large arrays. Using node.js on my (not so fast) computer mapping Array(1000000) with the string/split-method lasts 0.47 seconds, the array.apply method for the same crashes with a RangeError ( Maximum call stack size exceeded ).

You can also write a 'static' Array method to create an Array with n values and all usable/applicable Array methods in place:

Array.create = function (n, mapCB, method, initial) {
  method = method in [] ? method : 'map';
  var nwArr = ( function(nn){ while (nn--) this.push(undefined); return this; } )
                .call([], n);
  return mapCB ? nwArr[method](mapCB, initial) : nwArr;
};
// usages
Array.create(5, [callback], 'reduce', '');
Array.create(5).reduce( [callback], '');
Array.create(5).map( [callback] ).reduce( [callback] );
// etc.

Answer 2

As others have stated, the implementation of reduce causes this to throw a TypeError because the calling array has no initial values. To initialize an array with initial values of undefined and successfully invoke the function, try something like this:

Array.apply(undefined, Array(5)).reduce(function(cur,prv,n,a){alert(n)},'');