Avoiding the void type in generators in Typescript requires implementing proper typing for the

Question

Avoiding the void type in generators in Typescript requires implementing proper typing for the

As I work with a code generator that produces strings, I'm encountering this issue:

const pageUrl = generator.next().value;
const result = catalogPageParser(pageUrl);

An error is being thrown:

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

To address this problem, one option is to adjust the variable like this:

const pageUrl = generator.next().value ?? "some valid url";

Alternatively, the signature of the catalogPageParser function could be altered. However, neither of these solutions seem ideal to me. Are there any other suggestions for correctly typing this?

I am hesitant to use a default value as there is no suitable default in my scenario. Furthermore, allowing void as a valid argument for the function doesn't seem appropriate either.

Example

function* someGenerator() {
  yield "url1";
  yield "url2";
}

function acceptsStringOnly(argument: string) {
  console.log(argument);
}

const value = someGenerator().next().value;
acceptsStringOnly(value);

typescript

Answer 1

Answer №1

The reason why an iterator may not have a value is because it has reached its end. In such cases, the done property is set to false. TypeScript is able to narrow down the type of the value property if you confirm that done is false:

function* myGenerator() {
    yield 'first string';
    yield 'second string';
    yield 'third string';
}

const myIterator = myGenerator();
const myResult = myIterator.next();

let myString: string;

// This will fail in your scenario
myString = myResult.value;

if (!myResult.done) {
    // This will work
    myString = myResult.value;
} else {
    // Handle the situation when the iterator has ended
}

One approach to handle this situation is: 1) check if done is true first and take appropriate action (exit early, throw an error, etc.); 2) if you are confident that the iterator will always provide a value, you can use a non-null assertion.

Using ?? in your current solution is likely effective, but its suitability depends on your specific requirements. One potential drawback could be the lack of a proper default value in that scenario. In general, addressing the scenario where the iterator has ended is a more robust approach.

Answer 2

The reason why an iterator may not have a value is because it has reached its end. In such cases, the done property is set to false. TypeScript is able to narrow down the type of the value property if you confirm that done is false:

function* myGenerator() {
    yield 'first string';
    yield 'second string';
    yield 'third string';
}

const myIterator = myGenerator();
const myResult = myIterator.next();

let myString: string;

// This will fail in your scenario
myString = myResult.value;

if (!myResult.done) {
    // This will work
    myString = myResult.value;
} else {
    // Handle the situation when the iterator has ended
}

One approach to handle this situation is: 1) check if done is true first and take appropriate action (exit early, throw an error, etc.); 2) if you are confident that the iterator will always provide a value, you can use a non-null assertion.

Using ?? in your current solution is likely effective, but its suitability depends on your specific requirements. One potential drawback could be the lack of a proper default value in that scenario. In general, addressing the scenario where the iterator has ended is a more robust approach.

Avoiding the void type in generators in Typescript requires implementing proper typing for the

Example

Answer №1

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