Why generics are not working as expected? about typescript

[whzx5byb]

Shouldn't TypeScript know that if authenticated is true, the level prop should have an id property instead of a name property?

No, ts won't do type narrowing on props if it is not a discriminated union.

but I want to avoid retyping the props multiple times.

You can avoid retyping by using distribution behavior, in variant ways.

A. distribute on original type Props<T>:

- type Props<T extends boolean> = {
+ type Props<T extends boolean> = T extends unknown ? {
    authenticated: T;
    level: T extends true ? {id: number} : {name: string};
- };
+ } : never;

or

B. distribute when pass in function parameter

- const User = <T extends boolean>(props: Props<T>) => {
+ const User = <T extends boolean>(props: T extends unknown ? Props<T> : never) => {

Both ways will work for you.

[jcalz]

This is a question and unfortunately doesn't belong here (when you opened a new issue you chose "Other", but you really should have chosen "Question", which tells you to go to Stack Overflow instead.) I'm still waiting for you to respond to the comment requesting clarification at your Stack Overflow question, which is the right place for this discussion.

[RyanCavanaugh]

It looks like this is a question rather than a bug report. This issue tracker is for tracking bugs and active work on TypeScript itself, rather than a general forum for programmers using TypeScript to get help or ask questions.

You can ask questions on sites like Stack Overflow. We are not able to provide one-on-one support on the issue tracker. Please read the issue template carefully - it has important information on what kinds of reports can be acted on here, as well as links to useful TypeScript resources. Thanks!