filter noUncheckedIndexedAccess with own property check

[cookiesjuice]

Search Terms

ownProperty

hasOwnProperty

noUncheckedIndexedAccess

type check

Suggestion

noUncheckedIndexedAccess is a great feature. However, ts reports that object can be undefined even with hasOwnProperty check.

I suggest that a hasOwnProperty check makes unchecked indexed accesses checked, and return the checked value.

Use Cases

const obj: {[key: string]: string} = {};
// do something with obj
const key = "someKey";
if (Object.prototype.hasOwnProperty.call(obj, key)) {
  console.log(obj[key].length); // TS2532: Object is possibly 'undefined'.
}

Since only string values are allowed in obj, the only case such value is undefined is when the key does not exist on obj. The value of obj[key] can only be string. However, typescript still reports type of obj[key] being string | undefined.

Examples

const obj: {[key: string]: string} = {};
// do something with obj
const key = "someKey";
if (Object.prototype.hasOwnProperty.call(obj, key)) {
  console.log(obj[key].length); // no problem, since obj[key] has correct type string
}

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.

[MartinJohns]

Related: #10530

[magwas]

Actually I consider the current behaviour to be a bug.

This playground reports two errors where I believe there should be none, as both cases are checked:

https://tsplay.dev/WG85Kw

Please tell me if I have to open a separate bug report either because it is a bug, or I am commenting to something unrelated.