Record from noUncheckedIndexedAccess: false can't be expressed when noUncheckedIndexedAccess: true

[safareli]

Suggestion

Like with ways for adding and removing optionality form properties ?/- there seams to be a need to remove the special undefinedness of properties introduced in noUncheckedIndexedAccess:true

🔍 Search Terms

noUncheckedIndexedAccess Record Array

✅ 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.

📃 Motivating Example

With noUncheckedIndexedAccess:false we have a notion of Record that defines values, for all the possible properties. In practice there are not many actual values like that. Point ofnoUncheckedIndexedAccess:true was to automatically add | undefined type to result of property access so we are more type-safe automatically.

The main TypeSafe use-case for the records which do have value for any possible key are Proxies. And looks like we have lost a way to express the those properly with noUncheckedIndexedAccess:true

💻 Use Cases

Imagine

const val = new Proxy<{[k in string]: string}>({}, {
  get: (_, p) => p.toString().toUpperCase()
});

const foo = val.foo

with noUncheckedIndexedAccess:false, typer of foo is string tho with noUncheckedIndexedAccess:true, foo has type: string | undefined which is not correct. here we know 100% that any value we read from val via property access, must be string, but it can't be expressed using Typescript when noUncheckedIndexedAccess:true.

We can visualize what we lost using this table:

| noUncheckedIndexedAccess:false | Record<string,string> | Record<string,string | undefiend> | 
| noUncheckedIndexedAccess:true  | _____________________ | Record<string,string>             |

[jcalz]

You note that, in practice, such proxy-all-the-things values are rare. That seems to imply that this suggestion would only rarely help people. Am I missing something?

[andrewbranch]

Duplicate of #47594

Also related: #46969 (comment)

[safareli]

that kind of proxy might not be that common tho opting out from noUncheckedIndexedAccess:true per type level us usefull. i.e. you want noUncheckedIndexedAccess:true for 99% of the types but for some you don't. good use case for this is importing class names from scss/css module file in react code

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.