TypeScript is not detecting the checks before usage

So I've been trying to figure out how can I get TS to see I'm checking my variables exist before I use them? Here's the code :

This is the function that checks

function envChecks(){
  if (!process.env.DRIVER_PATH) {
    throw new Error('Environment variable DRIVER_PATH is not set');
  }
}

This is how it's used :

export async function thisDoesSomething() {

  envChecks();

  const pathToDriver = path.resolve(process.env.DRIVER_PATH); <- This Line here is give me a TS error
  // ...more code here
}

This is the TS error :

Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
  Type 'undefined' is not assignable to type 'string'.ts(2345)

If I check before the usage (not in a function), it doesn't complain then. But I prefer cleaner code.

A very simple way is to have your check function actually return the properly typed data or throw. Start by defining an interface that describes the data you will be needing:

interface TheData {
  driverPath: string;
  foo: number;
  bar: boolean;
  ...
}

Now feed any input to the check function and have it return the interface as:

function envChecks(env: ProcessEnv, baz: SomeOtherType): TheData {
  if (!env.DRIVER_PATH) {
    throw new Error('Environment variable DRIVER_PATH is not set');
  }
  // Now this is properly typed to string, NOT string | undefined
  const driverPath = env.DRIVER_PATH;

  if (typeof baz.foo !== 'number') {
    throw new Error('baz.foo is not a number');
  }
  const foo = +baz.foo;

  // and so on...

  return {
    driverPath,
    foo,
    bar
  };
}

Using it is straightforward:

export async function thisDoesSomething() {

  const theData = envChecks();

  const pathToDriver = path.resolve(theData.driverPath);
  // ...more code here
}

There may be more complicated ways with type guards, but I believe this should do for your case.