Why {} != ( {} ) in JavaScript?


It's commonly known that {} is shorter way to define an object like [] is for an array.

But now I am wondering why:

{} != ({})
  • {} evaluates to undefined
  • ({}) evaluates to "correct" Object

Why is JavaScript behaving like this ?

For example 1 equals to (1), so why {} not equals to ({}) ?

{} != ({})

This is a syntax error.

SyntaxError: Unexpected token !=

{} is ambigious like that. Is it an empty block, or an empty object literal? It's failing because a comparison operator can not follow a block.

Wrapping it with parenthesis makes the parser treat it as an expression. An expression can't contain a block, so it knows it's an object.

However, if you wrap that comparison in an expression...

({} != ({}))

...it's still true because variables which have an object assigned to them's values are a pointer to them and as a consequence, they are never copied around by their contents (though this is irrelevant to your example). Because of this, their pointer is always different and the comparison fails.

This also implies that comparing two variables which point to the same object will work as expected, as their pointers will be the same.


{} != {} is true because both {}'s are completely different objects. However, variables that are referencing the same object hold equality:

?var a = {},
    b = a;

assert( a == b )?