Question

In JavaScript, I wonder why I can do this:

[].slice.apply([1,2,3], [0, 2]) // returns [1, 2]

Which is as far as I understand equivalent of this:

Array.prototype.slice.apply([1,2,3], [0, 2]); // returns [1, 2]

But I can’t do this:

{}.hasOwnProperty.apply(a, ['a']);

Which would allow to not type this long line:

Object.prototype.hasOwnProperty.apply(a, ['a']); // returns true or false

Well, while I am at it, I also see that I can actually do all of these:

''.indexOf.apply('asdasd', ['s']); // returns 1
true.toString.apply(true); // returns // "true" as string

What the deal with {} ? 🙂 Why is it special?

Answer 1

Is hasOwnProperty('a') a typo?

{}.hasOwnProperty.apply(a, ['a']);

But that doesn’t quite work due to JavaScript interpreting {} as block delimiters. Just add parentheses:

({}).hasOwnProperty.apply(a, ['a']);

Check it:

> var a = {'a': 1}
  Object
> {}.hasOwnProperty.apply(a, ['a']);
  SyntaxError: Unexpected token .
> ({}).hasOwnProperty.apply(a, ['a']);
  true