RegExp.prototype.exec() and test() parameters are optional

[newlukai]

TypeScript Version:
3.8.3

Search Terms:
RegExp

Code

import * as assert from 'assert';

let matches = /ab(c)/.exec('abc');
assert.ok(Array.isArray(matches));
assert.equal((matches as RegExpExecArray).length, 2);

matches = /ab(c)/.exec();
assert.equal(matches, null);

Expected behavior:
Compiles.

Actual behavior:
Doesn't compile due to an expected argument for exec in line 8. But the corresponding JavaScript code runs without error.

Playground Link:
http://www.typescriptlang.org/play/?ssl=8&ssc=29&pln=1&pc=1#code/JYWwDg9gTgLgBAKjgQwM4tagprOAzKCEOAcjW1hIG4BYAKHoBst4RkYBjACy3QF44AemQAjABQcAlIIB0WAB5YOYsiI4lJtOuRwwZEANZiAglCjIAnjOCpT5i2LaceqSZvo7YcgI4BXZIxijuzcvBhwAEpYAOYAovJg8Up2lpIyzAB20TBcADRwAEzuDHROofxCohLScorKxZ56WH4Bwc68+Rm+jIyaQA

Related Issues:
Did not find other bugs that looked similar.

[ilogico]

It runs without errors but it's certainly a mistake.

[IllusionMH]

While it works in JS, JS also has no problems with null, 123 and {a: 123} (doesn't throw an error), how implicit undefined is better?

Just use .exec('undefined') for same effect but at least it won be confusing when /und/.exec() != null and /und/.test() === true.

[RyanCavanaugh]

See https://stackoverflow.com/questions/41750390/what-does-all-legal-javascript-is-legal-typescript-mean

[newlukai]

I proposed this because I had an issue when using an optional chain:
/abc/.exec(someObject.someSubObject?.someValue);
What TypeScript defines as "JS is improper about this" forces me to write
/abc/.exec((someObject.someSubObject?.someValue as string)).

I'm wondering if that's intended. I could live with it, but I think it's an aspect where TS feels more challenging.

[ilogico]

Like @IllusionMH suggested, you might as well write /abc/.exec(someObject.someSubObject?.someValue ?? 'undefined'), which is probably not what you meant, but it's equivalent and more explicit.
That code is wrong and TypeScript is warning you about it. You're using casts to eliminate the warning instead of admitting and correcting the mistake :)
You probably mean something like someObject.someSubObject?.someValue != null ? /abc/.exec(someObject.someSubObject.someValue) : null

[newlukai]

I guess you are right.