another way of noUncheckedIndexedAccess

[LongTengDao]

Search Terms

Suggestion

tsconfig: noUncheckedIndexedAccess is a good new feature, but whole project level setting is too coarse-grained to use.

traditional { [index :number] :undefined | T } could control that in api level. but not like noUncheckedIndexedAccess, it makes arr[0] = undefined valid which is not intended.

I wait the feature so long, but when I open noUncheckedIndexedAccess, there are too many too many too many errors, which cause trouble far more than help.

so in fact, we need a feature that makes T-only when writing, and T | undefined when reading.

Use Cases

Examples

maybe something like:

type T = string;
const myArray :{
    optional [index :number] :T
    optional item :T
};
myArray[0]// T | undefined
myArray.item// T | undefined
myArray.item = undefined// error, undefined is readonly
if ( 'item' in myArray ) {
    myArray.item// T
}

? to be rethinked would be best, but it took too much historical burden.

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.

[LongTengDao]

nice in 4.4 via --exactOptionalPropertyTypes