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3.9 Regression Merge Types #38571
Description
TypeScript Version: 3.9.2
Search Terms: Regression, merge types
Code
type Merge<T, S> = Omit<T, Extract<keyof T, keyof S>> & S;
type WithNumberId = {
_id: number,
otherKey?: number,
};
type WithStringId = {
_id: string,
otherKey?: string,
};
function convertToStringId<T extends WithNumberId>(data: T): Merge<T, WithStringId> {
const convertedData: Merge<T, WithStringId> = {
...data,
_id: data._id.toString(),
};
if (data.otherKey) {
convertedData.otherKey = data.otherKey.toString();
}
return convertedData;
}Expected behavior: No error (3.8x is fine)
Actual behavior: Definition of const convertedData gives error
Related Issues: Maybe #38460
Activity
x87 commentedon May 14, 2020
This is expected due to a breaking change #37195
micheleangioni commentedon May 14, 2020
Are you sure Seemann (@x87) ? The NON optional _id property is causing the problem.
x87 commentedon May 14, 2020
Michele Angioni (@micheleangioni) the error says:
if you remove the
_idproperty from typesWithNumberIdandWithNumberStringaltogether the error remains.if you remove the
otherKeyproperty it's gone.RyanCavanaugh commentedon May 14, 2020
This is the correct behavior. At its initialization,
convertedDatahas a declared property of typestringinhabited (possibly) by a value of typenumber.micheleangioni commentedon May 15, 2020
Ok, thanks Ryan Cavanaugh (@RyanCavanaugh) for pointing out is was due to 'otherKey' :)