How does this function validate its input — does it throw on bad arguments?

Yes, it does basic input validation. The function checks that the argument is a string, and throws a TypeError if not:

export default function escapeStringRegexp(string) {
	if (typeof string !== 'string') {
		throw new TypeError('Expected a string');
	}
	...

[index.js:L1-L4]

This is a simple runtime typeof check — if you pass anything other than a string (e.g., a number, object, undefined, null), it throws TypeError: Expected a string immediately, before any escaping logic runs.

There's no test in test.js that explicitly exercises this error path (the visible tests only cover valid string inputs — special characters and the - handling, [test.js:L1-L24]), but the check is present in the source itself.

On the TypeScript side, the type definition just declares the expected signature:

export default function escapeStringRegexp(string: string): string;

[index.d.ts:L16-L17]

So under TypeScript, passing a non-string would be caught at compile time; the runtime TypeError guard in index.js is what protects plain JavaScript callers.

FOLLOW_UPS:

• What characters does the regex actually escape?
• Why is - handled differently with \x2d?
• Are there tests for invalid (non-string) input?