filter noUncheckedIndexedAccess with own property check

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.

Start with the provided TypeScript reproducer and inspect how noUncheckedIndexedAccess and Object.prototype.hasOwnProperty.call are handled by the type checker. Confirm the current diagnostic, then determine whether the indexed access can be narrowed without changing emitted JavaScript; done means the example type-checks with regression coverage for the behavior.