Detecting invalid JS strings

[proud tendon]

Is there any built-in way to "detect" invalid strings?

Here's an example:

// this is an invalid unicode code point
const bad = "\udc11"

// but i can console.log it:
console.log(bad)
// prints: �

// and i can use it in other strings:
const foo = bad + "-" + bad

// but when i try to evaluate it in the repl:
> badAndPrefixed
Unterminated string literal Unknown exception

Some questions:

1. Can I somehow detect "bad" Unicode strings?
2. Why can I console.log it, and what does it do?
3. What happens in the Deno REPL that makes it throw an error?

[wooden walrus]

There's a stage 3 proposal to detect non-valid-UTF-8 (or to use the term they're going with in the proposal, "non-well-formed") strings: https://github.com/tc39/proposal-is-usv-string

for now you can test it with !/\p{Surrogate}/u.test(str)

2. console.log() and the REPL have different printing implementations, with console.log() being written in JS and the REPL mainly in Rust. Rust's string type is UTF-8-based, and it doesn't support invalid UTF-16.

Looks like what console.log() is essentially doing is encoding the JS string into UTF-8 using the "lossy" encoding which turns invalid UTF-16 code points into a replacement character (U+FFFD, �)

I think there's a bug open for the REPL output to do that