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Iterating through object and assigning to its properties leads to 'never' #38735
Description
TypeScript Version: 3.9.2
Search Terms: Type is not assignable to type 'never'
Code
interface TraitsSummary {
name: string;
age: number;
gender: 'male' | 'female' | 'other';
}
type TraitsDetailed = {
[K in keyof TraitsSummary]: {
value: TraitsSummary[K];
common: boolean;
required: boolean;
}
}
const values: TraitsSummary = {
name: 'Default',
age: 0,
gender: 'other'
};
const details: TraitsDetailed = {
name: { value: 'Miranda', common: true, required: true },
age: { value: 33, common: true, required: true },
gender: { value: 'female', common: true, required: false }
};
const keys = Object.keys(details) as Array<keyof TraitsSummary>;
for (let i = 0; i < keys.length; i++) {
values[keys[i]] = details[keys[i]].value;
}Expected behavior:
TraitsSummary and TraitsDetailed have exactly the same keys.
values of props under the keys have matching types so there shouldn't be any error.
Actual behavior:
Type 'string | number' is not assignable to type 'never'.
Type 'string' is not assignable to type 'never'.(2322)
P.S.
I have dozens of errors like Type 'some_type' is not assignable to type 'never'.
All of them happens when I'm trying to assign to object property selected by dynamic key.
I fixed some of issues with generics
(<T, K extends keyof T>(obj: T, keys: K[]) => {
for (let index = 0; index < keys.length; index++) {
const key = keys[index] as keyof T;
filters[key] = null; // or other actions
}
})(obj, originalFields);
But that's not working when you need to map values of one object to another regarding keys are same.
Activity
jack-williams commentedon May 23, 2020
This behaviour was introduced here #30769, and has lots of relevant discussion.
DzmVasileusky commentedon May 25, 2020
Jack Williams (@jack-williams) Thanks, I'm aware of that change but it didn't give me clear explanation on how to adjust my code to it.
To prevent that code from failing I need a monstrous typescript constructions which decrease a code readability awfully. And sometimes it's just super hard to invent such a construction and I have to put
anyto save the day.typescript-bot commentedon Jun 13, 2020
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.