Creating x objects using array and map

Question

I want to create 10 Fly objects in a functional manner. I thought this would work:

var flies = new Array(10).map(function() {
    return new Fly();
});

It doesn't. It creates an array of length ten with undefined as their values.

How can I create 10 objects without using for(var i = 0; i < 10; i++)...?

EDIT: This is an academic exercise for the sake of learning only. It's find if a for is used under the hood. I'm just trying to figure out what JavaScript can do.

`for(var i = 0; i < 10; i++)` seems like the best way to solve this problem. Why do you not want to use it? — gen_Eric, Oct 08 '14 at 20:14
`[0,1,2,3,4,5,6,7,8,9].map` - if you're dead set against `for` — tymeJV, Oct 08 '14 at 20:15
Read the docs for [array.map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map): `callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes that are undefined, those which have been deleted or which have never been assigned values.` — Matt Burland, Oct 08 '14 at 20:16
In ES6 you'll be able to do: `Array.from({length: 10}, _ => new Fly());` — jmar777, Oct 08 '14 at 20:19
If it's a personal thing against `for`, you can use: `var i=0; while(i<10){ /*...*/; i++ }` - =P — LcSalazar, Oct 08 '14 at 20:26
This is a particularly wasteful approach, but it's at least interesting at the academic level: `new Array(10).join().split(',').map(function() { return new Fly(); });` — jmar777, Oct 08 '14 at 20:26

score 2 · Accepted Answer · answered Oct 08 '14 at 20:18

2

Out of the box, there is only @elcanrs way. If you use a library such as Underscore you could use http://underscorejs.org/#times

var flies = _.times(10, function(){return new Fly()})

answered Oct 08 '14 at 20:18

pdjota

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Guess what `_.times` uses to do this? Yep! A *for*: `for (var i = 0; i < n; i++) accum[i] = iteratee(i);` :-D – gen_Eric Oct 08 '14 at 20:20
Actually they use Array for creating an accumulator and then _.iteratee which uses more calls that a for() for sure http://underscorejs.org/docs/underscore.html – pdjota Oct 08 '14 at 20:22
I was just pointing out that the OPs restriction to not use `for` is silly since any other solution will probably just use it internally. – gen_Eric Oct 08 '14 at 20:23
Indeed, in JavaScript the best way may be a for not a _.times. – pdjota Oct 08 '14 at 20:26
2

@RocketHazmat: once we get TCO we won't need to use loops anymore! yay – elclanrs Oct 08 '14 at 20:28
What is TCO? Tail-call optimization? – core Oct 09 '14 at 14:57

score 1 · Answer 2 · answered Oct 08 '14 at 20:44

I'm pretty sure that any fancy approach will be using a for, or another loop, at a lower level... If you reeeealy want to get rid of this, you can work with a recursive approach and create your own Fly Factory function!

Honestly, IMO it's a waste of effort. But at least it has not for loop! =D

function Fly() {
}

function CreateFlies(n) {
    var arr = [new Fly()];
    arr = (n > 1 ? arr.concat(CreateFlies(n-1)) : arr);
    return arr;
}
var flies = CreateFlies(10);
alert(flies);

Creating x objects using array and map

2 Answers2