suggest `in` operator when accessing non-common property of union types

🔍 Search Terms

access to non-common property of union types, suggest in operator for union types

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Background: https://twitter.com/kentcdodds/status/1420125238436655104?s=21

When accessing props that are not on all union types, tsc gives Property 'foo' does not exist on type 'XX', which is confusing to newcomers. We can suggest the in operator when it is used inside if condition.

📃 Motivating Example

type A = {one: string}
type B = {two: string}
type C = A | B

declare const thing: C

// 😕Before:
// Property 'two' does not exist on type 'C'.
//  Property 'two' does not exist on type 'A'.(2339)
if (thing.two) {
  // it's a B
}

// 😄After:
// Property 'two' does not exist on type 'C'.
//  Property 'two' does not exist on type 'A'.(2339)
// Did you mean to use `'two' in thing`?
if (thing.two) {
  // it's a B
}

💻 Use Cases

まず、TypeScript コンパイラにおける診断 2339 と関連する提案診断の実装およびテストを見つけます。if 条件内でのプロパティアクセスの動作を確認し、次に union 型を対象とするテストとともに、提案された in 演算子の提案を追加して、診断の文言を確認します。