Why TypeScript ReturnType returned sequence is unstable in vscode

[Eve-Sama]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: 1.73.0 (Universal)
• OS Version: Mac 12.5.1

Steps to Reproduce:

type A = () => 'a';
type B = () => 'b';
type Res1 = ReturnType<A | B>;
type Res2 = ReturnType<B | A>;

It's weird that above code run in vscode. I think the Res1 and Res2 is different. But they always keep same output. Sometimes they are 'a' | 'b', sometimes they are 'b' | 'a'. I have to restart vscode window or restart ts server trying to get right output. Is this a vscode bug?

[VSCodeTriageBot]

Thanks for creating this issue! It looks like you may be using an old version of VS Code, the latest stable release is 1.73.1. Please try upgrading to the latest version and checking whether this issue remains.

Happy Coding!

[Eve-Sama]

It still exist in 1.73.1

[Eve-Sama]

I know they are same when used as union type. I wonder why the result is unstable? Different time display different sequence.

[Okaretkina7]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: 1.73.0 (Universal)
• OS Version: Mac 12.5.1

Steps to Reproduce:

type A = () => 'a';
type B = () => 'b';
type Res1 = ReturnType<A | B>;
type Res2 = ReturnType<B | A>;

It's weird that above code run in vscode. I think the Res1 and Res2 is different. But they always keep same output. Sometimes they are 'a' | 'b', sometimes they are 'b' | 'a'. I have to restart vscode window or restart ts server trying to get right output. Is this a vscode bug?

[Eve-Sama]

Yes, it occur when all extensions are disabled. And it occur in my friends.

[Eve-Sama]

It works fine in IDEA

[Dimava]

@Eve-Sama you didn't provide your TS version, and it's 3.x?

[Eve-Sama]

"typescript": "^4.8.3"

[Eve-Sama]

TypeScript version is 4.8.4 via package.json from node_modules

[Dimava]

try

type A = () => 'a';
type B = () => 'b';
type C = () => 'c';
type D = () => 'd';

type x = [
  // ^?
    ReturnType<A | B>,
    ReturnType<B | A>,
    ReturnType<D | C>,
    ReturnType<C | D>,
]

It seems that I can't make an array with different ones, but the order depends on which one was instantiated earlier

[Eve-Sama]

export type TestFn = <T extends ReadonlyArray<() => string>>(request: T, thenCallback: (data: ReturnType<T[number]>) => void) => void;
const fnA: () => 'a' = () => 'a';
const fnB: () => 'b' = () => 'b';
const fnC: () => 'c' = () => 'c';
const fnD: () => 'd' = () => 'd';
const fn: TestFn = (requestList, thenCallback) => {
  console.log(requestList, `requestList`);
  console.log(thenCallback, `thenCallback`);
};

fn([fnA, fnB, fnC, fnD], data => {
  console.log(data, `data`);
});

This bug lead to this trouble.

[IllusionMH]

TS doesn't provide any guarantees about union members order. Always been this way.

Basically duplicate of #17944

[Eve-Sama]

TS doesn't provide any guarantees about union members order. Always been this way.

Basically duplicate of microsoft/TypeScript#17944

But why random?

[Dimava]

@Eve-Sama not "random", but "instantiation order"
It's not sorted, and the first result is considered to be correct

[Eve-Sama]

If it's ts's feature, Why it works fine in IDEA?

[fatcerberus]

I think IDEA uses its own language service that's based on the official one but with enhancements/customizations so it doesn't always behave the same. Even so, this is just about display order - A | B and B | A are exactly the same type to the compiler.

[Eve-Sama]

const fnA: () => 'a' = () => 'a';
const fnB: () => 'b' = () => 'b';
const fnC: () => 'c' = () => 'c';
const fnD: () => 'd' = () => 'd';
const fn: TestFn = (requestList, thenCallback) => {
  console.log(requestList, `requestList`);
  console.log(thenCallback, `thenCallback`);
};

fn([fnA, fnB, fnC, fnD] as const, data => {
  console.log(data, `data`); // data: "a" | "b" | "c" | "d"
});

I have found how to make it works. Just need to add as const.