obj[prop] || (obj[prop] = {}): Object is possibly 'undefined' -- Documentation?

[therealgilles]

Suggestion

It would be great to have TS documentation about "Object is possibly 'undefined'. ts(2532)" common cases that most everyone will run into and be puzzled about. The TS playground below shows such an example when using code like: obj[prop] || (obj[prop] = {}) and when most everyone would expect obj[prop] not to be flagged as 'undefined'.

TS Playground: link

🔍 Search Terms

Object is possibly 'undefined'. ts(2532)
TypeScript Object property assignment
TypeScript Object OR assignment undefined

✅ 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.

⭐ Suggestion

I would like to see common cases documentation about the "Object is possibly 'undefined'. ts(2532)" error. Maybe it exists and I could not find it.

📃 Motivating Example

I see many issues filed related to ts(2532) because of the current limitations of TypeScript.

💻 Use Cases

See TS Playground link.

[fatcerberus]

This isn't really about TS2532 specifically but about type narrowing in general--TS doesn't treat any check involving obj[key] as a type guard unless key is a literal. This does come up often though, see e.g. #48227 or #48203, and is a known limitation of control-flow-based type narrowing so it should probably be mentioned in the FAQ at this point.

[MartinJohns]

Just pointing out that the issues on GitHub are considered to be part of the documentation.

[therealgilles]

@MartinJohns: Understood. I think a dedicated FAQ entry would be beneficial to the community in this case. It's not easy to browse through thousands of (open/closed) issues, trying to figure out if something is a bug or expected behavior.

[MartinJohns]

I think a dedicated FAQ entry would be beneficial to the community in this case.

Considering how often the issue pops up I absolutely agree. Why don't you go ahead? :-)

[therealgilles]

@MartinJohns: I will give it a go next week. I admit I am still fairly new to TS but I'll try to gather examples from the filed issues.