What do square brackets after a function reference mean?

Question

I just spent about 15 minutes debugging a piece of JavaScript code, and discovered the problem was that I had written

matches.push[[-1]];

instead of

matches.push([[-1]]);

like I intended, where matches is an array. Can somebody explain to me why JavaScript didn’t throw a syntax error on the former, and what its meaning is?

Answer 1

Why

matches.push is a Function Object, and you can access object properties and methods through dot notation or the bracket notation. Basically you're asking for something which isn't in the push Function Object, so it yields undefined.

Note

If you added something with the key [-1] as in matches[[-1]] = "something" it would also be valid, so the syntax is valid, simply not what you wanted to do.