Object entry remains typed as undefined after assignment

[Ionaru]

TypeScript Version: 3.7.0-dev.20190824

Search Terms:
Object is possibly undefined
TS2532

Code

interface IData {
  data: number;
}

interface IMyObject {
  [index: string]: IData | undefined;
}

const myObject: IMyObject = {};
const objectKey = 'something';

myObject[objectKey] = {
  data: 5
}

console.log(myObject[objectKey].data); // TS2532: Object is possibly 'undefined'

Expected behavior:
Code compiles without error.

Actual behavior:
Error: TS2532: Object is possibly 'undefined'. on myObject[objectKey]

Playground Link:
http://www.typescriptlang.org/play/index.html#code/JYOwLgpgTgZghgYwgAgJIBE5jsg3gKGWQBMs4AuZEAVwFsAjaAbnwF999RJZEVUBZAJ4B5egCsICMHkLIA2qGIQAHpQDOYKKADmAXUoYyyAD7JqIJTFARiLdvgQB7EBuS0R4yWANDREqcgAvHisLE4u0o6eUgDSEIJByADkao60EGAAFjpJLPjufl5yUf5gcYK6iQREpNiUAKxsHOGpADYQAHStjtoAFAXRYMWD5bodtXAAlExAA

Related Issues:
#13778, #17960, #26599, #29642

It seems related issues have been around for a long time, in addition issue #26599 was closed as a "wontfix". This is weird to me because it seems Typescript should be able to detect that objectKey has not changed between the object assignment and query.

[AnyhowStep]

If anyone else is looking for a workaround,

interface IData {
  data: number;
}

interface IMyObject {
  [index: string]: IData | undefined;
}

const myObject: IMyObject = {};
const objectKey = 'something';

myObject.something = {
  data: 5
}

console.log(myObject.something.data);

Playground

Just don't use string literals/constants. Use dot notation instead.

[Ionaru]

Just don't use string literals/constants. Use dot notation instead.

Good solution, but doesn't work when objectKey is a variable :)

What I posted is just a demonstration of the issue, in most cases the object keys are not constant but variables or parameters passed to a function.

[fatcerberus]

If the key is truly variable, then this is related to #31445: TS isn't equipped to understand that two mentions of obj[key] access the same property in both places; since it doesn't even know that, it doesn't make sense to perform any narrowing on it.

[sandersn]

Yep, duplicate of #31445 (and some others).