Why does `{} == null` give a SyntaxError?

Question

I can compare {} to true or false or itself, but comparison to null or undefined gives a syntax error. Is this because {} is an object value and not a reference? It feels weird that it would be a Syntax Error instead of some kind of runtime type error, or just work.

To clarify, I'm curious why this is a SyntaxError, mostly compared to doing {} == {} which is not only not a SyntaxError, but no error at all.

for some reason the REPL seems to treat it as a block statement there — Jonas Wilms, Nov 14 '18 at 18:34
When you put everything in parentheses, `({} == null)`, it returns false. Firefox doesn't like comparing it at all: `{} == {}` throws a syntax error, except if you put in parentheses. — joppiesaus, Nov 14 '18 at 18:41
https://stackoverflow.com/questions/9032856/what-is-the-explanation-for-these-bizarre-javascript-behaviours-mentioned-in-the — epascarello, Nov 14 '18 at 18:47
#3 in the dupe. {} is seen as a block statement that does not return anything so it is same as `undefined == null` — epascarello, Nov 14 '18 at 18:53
@espascarello nope. why does `{} === {}` evaluate to *false* then?! — Jonas Wilms, Nov 14 '18 at 18:54
Why would it be true @JonasWilms? Objects comparing never equals. and that is a block statement equal to an object. — epascarello, Nov 14 '18 at 18:56
@espascarello no it is not! That would be a SyntaxError as well. But why isn't it? For some reason the REPL seems to treat some values as "in expression context" and some not. — Jonas Wilms, Nov 14 '18 at 18:57
Why would it be a syntax error.... lol, glad my dupe was removed for you can prove it with an answer. lol — epascarello, Nov 14 '18 at 18:58
@epascarello yup, because it was no dupe, you just don't get the question — Jonas Wilms, Nov 14 '18 at 18:59
I've been playing with this in [Treehugger](https://ajaxorg.github.io/treehugger/test.html), an AST viewer. Enter the expression into TreeHugger, and notice it thinks the `{}` is a block start unless its in parenthesis. — , Nov 14 '18 at 18:59
@JonasWilms You are free to try it out yourself. [AST Explorer](https://astexplorer.net/) gives similar results. — , Nov 14 '18 at 19:02
@amy I just did and `{} == {}` results in an error as the first operand is missing. — Jonas Wilms, Nov 14 '18 at 19:04
@epascarello not sure how that matters. The last line isnt even evaluated, while the second last one is — Jonas Wilms, Nov 14 '18 at 19:06
@JonasWilms Okay? I didn't ask, but good to know. http://esprima.org/demo/parse.html also gives similar results. I think I'm running out of AST viewers. — , Nov 14 '18 at 19:07
@JonasWilms: Chrome uses a heuristic to evaluate code that starts with a `{` as an expression instead of a statement. It looks like that heuristic doesn't detect `{} == null`. — Felix Kling, Nov 14 '18 at 19:16
@JonasWilms: Sure. No idea how it works `¯\_(ツ)_/¯` . Might just be a bug. — Felix Kling, Nov 14 '18 at 19:22
@felixKling bergi linked the relevant source in the dupe, its just a regex to check for `{` and `}`, which is ... pragmatic.... — Jonas Wilms, Nov 14 '18 at 19:27

Jonas Wilms · Accepted Answer · 2018-11-14T19:23:50.350

There are two main contexts when parsing code: the expression context and the statement context. The body of a function for example is a statement context, the right side of an assignment is an expression context. To distinguish both makes sense to forbid things like:

 if( if(true) ) alert("nonsense");

Now REPL have a really difficult task: On the one hand, they have to parse functions and codeblocks that get entered, and on the other hand you sometimes just want to check how a certain object look like. Therefore this:

 { a: 1, b: 2 }

is actually a SyntaxError in JavaScript, as a { in a statement context starts a block of code, and : is invalid then. However the REPL is clever enough and puts that object into an expression context, and evaluates it as if it would be in parens:

({ a: 1, b: 2 })

the same happens for:

 {} == {}

which is actually also a SyntaxError, however the REPL also moves it into an expression context:

 ({} == {})

That "moving into an expression context" is a complicated task, and it seems as if the REPL just does not see the expression here:

 {} == null

and therefore parses {} as a block. Thats the case because the REPL just naively checks if the first and the last character are { and }, which is the case for {} == {} but not for {} == null.

Relevant part of the chromium sourcecode:

 if (/^\s*\{/.test(text) && /\}\s*$/.test(text))
  text = '(' + text + ')';

executionContext.evaluate(text, "console", !!useCommandLineAPI, false, false, true, printResult);

I'm really glad someone else was as interested in this dumb bit of REPL weirdness as I was :) — Christopher Wirt, Nov 14 '18 at 19:31
@christopher I'm really surprised that there are so many duplicates ... I never heard of this behaviour before .... — Jonas Wilms, Nov 14 '18 at 19:33
mad props for finding the relevant bit of chromium sourcecode — Caleb Jay, Nov 14 '18 at 22:41

Why does `{} == null` give a SyntaxError?

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