best way to generate empty 2D array

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

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.

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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.

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(() => [])