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OptionalPropNames<T>: unexpected result in conditional type #21989
Description
TypeScript Version: 2.8.0-dev.20180216
Code
// Type-level filters to extract just the required or optional properties of a type
// Defns from https://github.com/Microsoft/TypeScript/pull/21919#issuecomment-365491689
type OptionalPropNames<T> = { [P in keyof T]: undefined extends T[P] ? P : never }[keyof T];
type RequiredPropNames<T> = { [P in keyof T]: undefined extends T[P] ? never : P }[keyof T];
type OptionalProps<T> = { [P in OptionalPropNames<T>]: T[P] };
type RequiredProps<T> = { [P in RequiredPropNames<T>]: T[P] };
// Let's extract just the optional property names in a type
type PropNames = OptionalPropNames<{req: string, opt?: number}>; // Names = "opt" | undefinedExpected behavior:
PropNames = "opt"
Actual behavior:
PropNames = "opt" | undefined. How did that undefined get in there?
Related Issues:
Noticed this while writing up #21988, but wasn't sure if the cause was related or not.
The RequiredProps and OptionalProps definitions come from #21919 (comment)
Activity
yortus commentedon Feb 20, 2018
I worked it out. This is a result of the way mapped types carry over modifiers (
readonly,?) from the source type to the mapped type. This can be addressed using the new-?syntax added in #21919: