Why does {} == false throw an exception?

Question

In IE and Chrome, typing this into the JavaScript console throws an exception:

{} == false   // "SyntaxError: Unexpected token =="

However, all of these statements are evaluated with no problem:

false == {}   // false

({} == false) // false

var a = {};
a == false    // false

Is this intentional behavior? Why does this happen?

Swanee

Another oddity: [] == false // true – Batha

@DexterW I was reading up on truthy values. Confusing stuff. – Swanee

Weird, [] == false is throwing me, but I feel like I've seen it before. Edit: Aha! The array's toString method is secretly called, producing '', which is falsey! Correspondingly, [] == '' is true. – Sitnik

Why does function a() { b: 1 } NOT throw an exception? – Sinew

@torazaburo I would like to know. – Swanee

@torazaburo in function a() { b: 1 } the statement b: 1 is defining a label b. See this answer: https://mcmap.net/q/127851/-what-does-39-39-colon-do-in-javascript and the EcmaScript spec section 12.12 – Vally

@BenjaminWegman Wow. Thanks for that. It's so strange that {b: 1} == false also throws an exception. I guess they have to get rid of ambiguity in the grammar one way or another... – Swanee

@Swanee I'm a little late coming back, but the { } in torazaburo's function definition is not acting like the { } in your {b: 1} == false. His is defining the function body block, whereas yours are the same as {} == false (a generic code block rather than an object). In your example, remember that (as Benjamin Wegman notes) b: 1 is creating a label in your generic code block, and not an object property. – Sitnik

Here's another interesting one: twitter.com/konklone/status/475819362172280833 – Swanee

Answer 1

In the console, when you start a statement with {}, you are not creating an object literal, but a code block (i.e. the same block as you would make with an if statement or a loop body). A symbol like == is then obviously not expected afterwards.

If you think of a code block, you know that something like a = 5; could come after it:

if (some_condition) {
    // do something
}
a = 5;

You can then use this to test in the console, and find that it works just fine:

{} a = 5;

Sitnik

That is very, very interesting. So.. let me get this straight. I know that JavaScript functions, arrays, and objects are very similar. You can add properties/functions onto just about anything. Are code blocks ({ ... }) perceived differently? Why can you not compare a code block with an object? Or is that even the issue? – Swanee

@Swanee A code block isn't any sort of object or data model. It's just language syntax, and nothing more, meant to show an inline logical separation of code. It's weird here because it's legal to have code blocks not associated with any sort of language construct like if or for or while or function(). For instance, try a=5; { b=6; } c=7; in your console, and then see what the values of a, b, and c are. This goes back at least as far as C syntax rules, which allows independent code blocks. – Sitnik

Thanks for your answer. You saved me some sleep. – Swanee

I think that should be "start a statement with…", but otherwise you're spot on. – Hogen