Question

Why is it that typing this in the console:

{}.toString
{}.hasOwnProperty
{}....

throws an SyntaxError, and typing:

[].slice
[].filter
({}).toString
({}.toString)

doesn’t?

What is the difference between the Array and the Object?

Answer 1

The problem is the ambiguity of the { symbol. Try this:

({}.toString())

When the parser sees {} at the very beginning of a new statement, it has to choose between

• Is this an object literal?
• Is this a statement block?

It always chooses the second one, a statement block. Thus {} is an empty statement block, and .toString() right after that makes no sense.

There’s no ambiguity with [] — an [ at the beginning of a statement can only be the start of an array literal (as part of an expression statement). However there are similar problems with the function keyword, which does double-duty as the start of a function declaration statement and the start of a function instantiation expression.