microsoft / microsoft/TypeScript
Generic derived value type
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Description
Hey guys.
I can not see if I'm doing anything wrong here. I am expecting to be able to get exact type from generic type ...
Is this just limitation of typescript or i am doing anything whrong here?
TypeScript Version: 3.1.3
Search Terms:
Generic derived value type
Code
export type filterTypeName = 'price' | 'price_range';
export type priceFilterValue = {
from: number;
to: number;
};
export type priceRangeFilterValue = {
fromRange: number;
toRange: number;
};
export type filterValueType<T extends filterTypeName> =
T extends 'price' ? priceFilterValue
: T extends 'price_range' ? priceRangeFilterValue : never;
type productFilterParameters<T extends filterTypeName> = {
filter: T;
value: filterValueType<T>;
};
function addProductFilter<T extends filterTypeName>(params: productFilterParameters<T>) {
switch (params.filter) {
case 'price':
const priceFrom = params.value.from; //Property 'from' does not exist on type 'filterValueType<T>'
break;
default:
break;
}
}
// parameter value is derivered based on filter type... not working id function definition
addProductFilter({ filter: 'price', value: { from: 1, to: 2 } });
//expected behavior something like this
type productFilterParametersCorrect = {
filter: 'price';
value: priceFilterValue;
} | {
filter: 'price_range';
value: priceRangeFilterValue;
}
function addProductFilterCorrect(params: productFilterParametersCorrect) {
switch (params.filter) {
case 'price':
const priceFrom = params.value.from; // correct
break;
case 'price_range':
const priceRangeFrom = params.value.fromRange; // correct
default:
break;
}
}
Expected behavior:
Code
const value: priceFilterValue
Actual behavior:
Code
const value: filterValueType<T>
Playground Link:
Playground
Related Issues:
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked Playground and the generic addProductFilter example using TypeScript 3.1.3, then compare the generic parameter behavior with the explicitly defined union. Done means the switch on params.filter provides the corresponding value properties without the reported error; the payload names no repository files or tests.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100