Array Destructuring Inference Is Incorrect #36635
Description
Search Terms
array destructuring, array destructuring inference
Suggestion
When you use array destructuring the type is inferred incorrectly. I hope something can be changed to enforce proper type inference when using this. In my code, TS has helped clean and prevent a lot of mistakes. The only mistakes that I repeatedly find being made either by me or others in my project are always related to this. Time and time again I have had these errors happen in runtime making the benefits of Typescript lost. An option to enforce this would be amazing, please. (Especially if it is just included inside the strict option.)
Use Cases
const example = (args: string[]) => {
const [userID, duration, ...reason] = args
// userID and duration is AUTOMATICALLy inferred to be a string here.
// However, if for whatever reason args is an empty array userID is actually `undefined` and NOT a `string`.
// This is valid but it should not be because userID could be undefined
userID.toUpperCase()
}Examples
const example = (args: string[]) => {
const [userID] = args
// userID is AUTOMATICALLy inferred to be a string here.
// However, if for whatever reason args is an empty array userID is actually `undefined` and NOT a `string`.
// Typescript SHOULD warn that this is not possible on undefined
userID.toUpperCase()
}Proper Way:
const example = (args: string[]) => {
const [userID, duration, ...reason] = args
if (!userID) return
// This is valid but it should not be because userID could be undefined
userID.toUpperCase()
}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, etc.)
- This feature would agree with the rest of TypeScript's Design Goals.