Condensing a sparse array in Javascript?

Question

I have an array of elements where the entries are sparse. How can I easily condense the sparse array into a dense array so that I don't have to keep checking for null and undefined values every time I loop through the data?

Here is some example data:

var sparse = [];
sparse[1] = undefined;
sparse[5] = 3;
sparse[10] = null;

var dense = sparseToDenseArray(sparse);
// dense should be [3]

Answer 1

In ES2017 (ES8) this is as easy as Object.values(sparseArray)

For example:

const sparseArray = [, , 'foo', 'bar', , 'baz', ,];
const compactArray = Object.values(sparseArray);
console.log(compactArray);

Note though that this method only removes gaps, shifting down the indexes of existing array elements as required. It does not remove elements explicitly set to undefined or null.

Answer 2

You can use filter() which is compatible with Firefox, Chrome, IE 9, Opera, and Safari web browsers.

According to David Flanagan, in Javascript: The Definitive Guide, an easy way of transforming a sparse array to a dense array is to use a filter on it like so:

var dense = sparse.filter(function (x) { return x !== undefined && x != null; });

This works since filter() skips missing elements and only returns true if x is not undefined or null.

If filter() is not supported, this will compact a sparse array:

var compacted = [];

for(var i = 0; i < sparse.length; i++)
    if(i in sparse)
        compacted.push(sparse[i]);

An exact equivalent of the filter() example is:

var compacted = [];

for(var i = 0; i < sparse.length; i++)
    if(sparse[i] != null)
        compacted.push(sparse[i]);