best way to generate empty 2D array

Question

is there a shorter, better way to generate 'n' length 2D array?

var a = (function(){ var i=9, arr=[]; while(i--) arr.push([]); return arr })();

a // [ [],[],[],[],[],[],[],[],[] ]

** old-school short-way**:

var a = (function(a){ while(a.push([]) < 9); return a})([]);

UPDATE - Using ES2015

Array(5).fill().map(a=>[]); // will create 5 Arrays in an Array
// or
Array.from({length:5}, a=>[])

Emptying 2D array (saves memory rather)

function make2dArray(len){
    var a = [];
    while(a.push([]) < len); 
    return a;
}

function empty2dArray(arr){
    for( var i = arr.length; i--; )
      arr[i].length = 0;
}

// lets make a 2D array of 3 items
var a = make2dArray(3);

// lets populate it a bit
a[2].push('demo');
console.log(a); // [[],[],["demo"]]

// clear the array
empty2dArray(a);
console.log(a); // [[],[],[]]

Answer 1

Another way:

for(var a = [];a.length < 10; a.push([])); // semicolon is mandatory here

Yet another way:

var a = []; while(a.push([]) < 10);

This works because .push() ^[docs] (specification) returns the new length of the array.

---

That said, this is the wrong way of "reducing code". Create a dedicated function with a meaningful name and use this one. Your code will be much more understandable:

function get2DArray(size) {
    size = size > 0 ? size : 0;
    var arr = [];

    while(size--) {
        arr.push([]);
    }

    return arr;
}

var a = get2DArray(9);

Code is read much more often than written.

Answer 2

Just discovered another ES6 way with one line expression:

Array.from({length: N}, () => [])

Array.from(arrayLike[, mapFn[, thisArg]])

More detail about its implementation/polyfill ⇢ MDN Array.from()

Yet another neat solution with help of array spread syntax:

[...Array(N)].map(() => [])

Answer 3

Array(cardinality).fill(0).map(function(item) {return [];});

where cardinality is the number of items you are looking at. In this case it would be 9. This was suggested by one of my colleagues actually. This is neat, I think :) This is valid from ECMA V6. Documentation: Array::fill