TypeScript seems to be failing to detect the necessary checks before they are used

Question

TypeScript seems to be failing to detect the necessary checks before they are used

I've been pondering on how to ensure TypeScript acknowledges that I am verifying the existence of my variables before using them. Below is the code snippet :

Here's the function responsible for these checks:

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

Now, let me show you how it's implemented:


export async function performTask() {

  verifyEnvVars();

  const driverPath = path.resolve(process.env.DRIVER_PATH); <- This line triggers a TS error
  // ...additional code here
}

This is the TypeScript error I encounter:

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

Interestingly, if I conduct the check directly rather than within a function, TypeScript doesn't complain. However, I prefer maintaining clean and organized code.

typescript

Answer 1

Answer №1

One effective approach is to ensure that your check function returns the correctly typed data or throws an error. To begin, create an interface that defines the necessary data:

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

Then, pass any input to the check function and have it output the defined interface like so:

function validateEnvironment(env: ProcessEnv, baz: SomeOtherType): NeededData {
  if (!env.DRIVER_PATH) {
    throw new Error('DRIVER_PATH environment variable is missing');
  }
  
  const driverPath = env.DRIVER_PATH;

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

  return {
    driverPath,
    foo,
    bar
  };
}

Implementing this method is simple:

export async function performAction() {

  const requiredData = validateEnvironment();

  const driverPath = path.resolve(requiredData.driverPath);
  // ...additional code here
}

This approach may seem basic compared to using type guards, but it should suffice for your scenario.

Answer 2

One effective approach is to ensure that your check function returns the correctly typed data or throws an error. To begin, create an interface that defines the necessary data:

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

Then, pass any input to the check function and have it output the defined interface like so:

function validateEnvironment(env: ProcessEnv, baz: SomeOtherType): NeededData {
  if (!env.DRIVER_PATH) {
    throw new Error('DRIVER_PATH environment variable is missing');
  }
  
  const driverPath = env.DRIVER_PATH;

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

  return {
    driverPath,
    foo,
    bar
  };
}

Implementing this method is simple:

export async function performAction() {

  const requiredData = validateEnvironment();

  const driverPath = path.resolve(requiredData.driverPath);
  // ...additional code here
}

This approach may seem basic compared to using type guards, but it should suffice for your scenario.

TypeScript seems to be failing to detect the necessary checks before they are used

Answer №1

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