I've been fiddling with JavaScript, and I found some behavior I don't understand. The problem seems to be with automatic semicolon insertion (ASI), but, to me, it looks like ASI is misbehaving. The following code has been tested with node v8.10, and with Google Chrome's developer tools' console (Version 73.0.3683.86 (Official Build) (64-bit)).
> 0 == {}
false
> {} == 0
...
The three dots mean Uncaught SyntaxError: Unexpected token == (taken from chrome's developer tool's console output.) It seems to be explained by ASI actually turning it into {;} == 0, but from reading the specs (linked above), it shouldn't do that. Did I misunderstand the specs?
There are some funny situations that this causes, and some weird ones too. For example, {} + 4 evaluates to 4, because it is {;} + 4, or simply +4. But, 4 + {} is '4[object Object]'. And, weirdly:
> {} + 4 + {}
'[object Object]4[object Object]'
It didn't become {;} + 4 + {}, which would be the same as 4 + {}, now it actually accepted the first {} as an object instead of a block!!
The right-most token matters:
> {} + 4 + {} + 4
'4[object Object]4'
Now, the first {} was interpreted as an empty block again. Furthermore,
> {} + {}
'[object Object][object Object]'
> {} + {} + 4
NaN
The second line's behavior seems to be ASI again, because +{}, or Number({}), evaluates to NaN (The first {} is interpreted as an empty block again.)
extra funny code:
> {a:3}.a + 2
... // Uncaught SyntaxError: Unexpected token .
> ({a:3}.a + 2)
5
This time, the first one actually becomes an empty block with the label a!
I expected no ASI to happen in those examples, since it should already be (I think) a valid production of the grammar. And it seems weird how we can keep alternating the right-most token ({} + 4 + {} and {} + 4 + {} + 4) and it changes the meaning of the first {}.