Javascript: Square brackets after function call

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.

Answer 2

Its just a property accessor :

 var matches = []; matches.push[1] = 'bla' document.write(matches.push[1]); 

Basically you do the following:

matches.push[[-1]]; 

resolves to (a single number in brackets becomes a string ) and while the accessor for objects is a string , you get

matches.push['-1']

and that resolves to

undefined

because the property '-1' is undefined.

Answer 3

Everything in js is an object, even a function. I imagine the engine just referenced a (nonexistent) field to the push function/object. That results in undefined .