JSON.parse returns evaluation errors instead of throwing them

[nicolo-ribaudo]

Given a variable let input = "[" + "0,".repeat(2**32 - 1) + "0]", this code:

JSON.parse(input)

is defined to return a RangeError (for the invalid array length, as defined in steps 3-5 of https://tc39.es/ecma262/#sec-arraysetlength), rather than throw it.

This is because instep 7-9 of https://tc39.es/ecma262/#sec-json.parse we always get the completion value without actually checking if it's a normal completion or a throw completion.

This is not actually observable in the implementations I could try, since all of them don't allow creating strings that long.

[jmdyck]

Note that if a spec algorithm says to (e.g.) "throw a SyntaxError", that's just a shorthand for returning a particular throw completion. (See 5.2.3.2 Throw an Exception.) There's no semantic difference between returning a throw completion via 'return' (or '?') vs returning it via 'throw'.

[nicolo-ribaudo]

Ok well, let me reword :)

"When Evaluate returns a throw completion, JSON.parse returns a normal completion containing the throw completion's value"

[jmdyck]

Hm. It seems like step 2 is meant to ensure that step 7's completion is normal. But ECMA-404 doesn't limit the length of an array structure, so your example isn't caught there.

And step 6 asserts that the ParseText in step 4 has no errors. But ArrayLiteral doesn't have an EarlyError if it contains (recursively) too many elements, so again your example passes through.

So yeah, it seems like JSON.parse needs to better handle the possibility that step 7's completion is abrupt.