fix: validator spec functions return their generic type

[harrim91]

I've been having an issue where the validator spec functions (str()etc.) don't return the correct type when passed a generic.

An example:

import { cleanEnv, str } from 'envalid';

type LogLevel = 'info' | 'error';

const env = cleanEnv(process.env, {
  LOG_LEVEL: str<LogLevel>(),
});

The return value of cleanEnv here is Readonly<{ LOG_LEVEL: string } & CleanedEnvAccessors>.

I would expect it to be Readonly<{ LOG_LEVEL: LogLevel } & CleanedEnvAccessors>.

If you add in a generic to cleanEnv, it highlights the problem further:

import { cleanEnv, str } from 'envalid';

type LogLevel = 'info' | 'error';

type Env = {
  LOG_LEVEL: LogLevel;
};

const env = cleanEnv<Env>(process.env, {
  LOG_LEVEL: str<LogLevel>(),
});

The line LOG_LEVEL: str<LogLevel>() gives a TypeScript error:

Type 'ValidatorSpec<string>' is not assignable to type 'ValidatorSpec<LogLevel>'.
  Type 'string' is not assignable to type 'LogLevel'.

So str<LogLevel>() is returning type ValidatorSpec<string> when I would expect it to return ValidatorSpec<LogLevel>.

This PR fixes the issue by just passing the generic type into the underlying makeValidator function so that the returned validator spec has the correct type.

[af]

Hi @harrim91 thanks for the PR! I can't figure out why the type T is not being inferred correctly by makeValidator()– when I tested #146 I'm quite sure it was working, but that was some months ago now. Which version of TS are you using?

I'll give this a spin on your same TS version and see if I can repro. Either way the diff looks sensible

[harrim91]

Hi @af

It's been happening for a while for me on various projects. I've been using type assertions to get around it, but thought I would take a deeper look. When I installed my local clone of the project with my changes, it fixed the issue.

The example I gave, I tested with 4.5.5.

tsconfig.json:

{
  "compilerOptions": {
    "target": "es2015",
    "module": "commonjs",
    "strict": true,
    "esModuleInterop": true
  }
}

[af]

Hey @harrim91 I was not able to repro the error you're seeing on latest main. Could you install 7.3.0-beta.1 (just published!) and let me know if that works better for you?

[af]

I think https://github.com/af/envalid/pull/194 solves the same problem here, if you're still seeing the problem please open a new issue 🙏