JS files should parse as JS, not as JSX

Bug Report

🔎 Search Terms

TS1005 allowJS

🕗 Version & Regression Information

3.3.3

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about allowJS and TS1005
• I was unable to test this on prior versions because playground

⏯ Playground Link

https://www.typescriptlang.org/play?jsx=0&ts=3.3.3&filetype=js#code/C4JwrgpgPABA9AHQEZwHTAgZ2ACgER4CU8cMA8gNIBQokUiK6WuBxcpAYgIICSAMlSA

💻 Code

tsc --allowJs --checkJs test.js

true< /\b/.test("") // OK
true</\b/.test("") // FAIL

This is a small reproducing example. The real-world code causing this issue is: https://unpkg.com/froala-editor@4.0.3/js/froala_editor.min.js

🙁 Actual behavior

:2:2 - error TS1005: ';' expected.
:2:7 - error TS1127: Invalid character.
:2:10 - error TS1109: Expression expected.

🙂 Expected behavior

Compiles successfully, because line 1 and line 2 are semantically equal in JS.

[DanielRosenwasser]

Also, keep in mind that the issue is not that the code compares something with a regex (implausible), but rather that the original code is effectively -1 < /someRegex/.text(someStuff) (plausible).

[dcsaszar]

I’ve had a look at the code and the trick used by the optimizer is that here -1 < makes the following && expression execute unconditionally: -1</x/g.test("y")&&console.log("OK") In other words, it disposes the return value of /someRegex/.test(someStuff) to make the code shorter.

There’s only one number-typed property

In theory, there are other (maybe unlikely in the real world) ways for “number < regex”, for example, 4</4/g.test("5").toString().length.