Is there a simple way to make these two lines into a one-liner?

Question

Here are the two lines:

pixels[x-left] = {};
pixels[x-left][y] = true;

It would greatly allow me to clean up some code if the two could be combined into a single expression.

My first attempt looked something like this:

pixels[x-left] = { y: true };

However the letter y is being used as the index instead of the value of y, which is not OK.

Answer 1

I'll start right off saying that Bergi's solution should be preferred, but in case you don't mind killing your code's readability:

(pixels[x-left] = {})[y] = true;

Fiddle

This works as an assignment expression returns the assigned value (in this case, a reference to the assigned object).

Answer 2

No, there is not no clean one*. Any property name in an object literal is interpreted literally.

One way out would be some kind of helper function, like

Object.set = function(o, p, v) { o[p] = v; return o; };

pixels[x-left] = Object.set({}, y, true);

but then everybody reading your code would need to know about Object.set.

_{* After @Fabricio's answer I can't state "no possibility" any more :-)}