Comparing a literal string union variable to `typeof` throws a `valid-typeof` error

[AnInternetTroll]

When I run deno lint --unstable on this typescript I am getting a error

function test(typeTest: "string" | "number" | "boolean", input: any) {
	if(typeTest === typeof input)
		console.log("Yey")
}

(valid-typeof) Invalid typeof comparison value
	if(typeTest === typeof input)
     ^^^^^^^^
    at file.ts:1:64

[magurotuna]

Thanks for the report! The linter should check typeof comparison only when it's compared to string literals, not variables. I'll fix it.

[magurotuna]

@bartlomieju Looking at #38, which is the PR that introduced valid-typeof rule, there was a discussion that you adopted a stricter option i.e. "requireStringLiterals": true over the default option in ESLint (actually the default is false).

It seems like @AnInternetTroll's example shows one of the useful and practical usages of typeof comparison with what's not string literals, although typeof foo === undefined still must be a mistake by a programmer in almost all cases. So I'd say it would be great if our valid-typeof behaved as follows:

valid

typeof foo === 'string';
typeof foo == 'undefined';
typeof foo === someVariable; // comparison with normal variables is okay

invalid

typeof foo === 'strign'; // this is typo

// comparison with the literal of `undefined`, `null`, or `Object` are invalid!
typeof foo === undefined;
typeof foo === null;
typeof foo === Object;

Comparison with undefined, null, Object literals are treated as errors, but with other variables are okay. This is a behavior of our own that is different from ESLint.

Any thoughts on this?

[ghost]

Comparison with undefined, null, Object literals are treated as errors, but with other variables are okay. This is a behavior of our own that is different from ESLint.

Of the three items mentioned, only null is a literal, the others are global properties, does that complicate anything?

Will comparisons against all global properties be disallowed? Hopefully, they are, comparing against any of them wouldn't make sense.

[bartlomieju]

@magurotuna sorry I completed missed the ping, I agree with your proposed solution.