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TypeScript shouldn't assume {[name: string]: NonUndefinedType} has no undefined properties #23817
Description
TypeScript Version: 2.8.3
Search Terms: object type undefined
Code
interface Foo {
[_: string]: number
}
const foo: Foo = {}
// foo.abc is undefined, but TypeScript allows this
foo.abc.toString()Expected behavior:
Object is possibly undefined
Actual behavior:
TypeScript allows it
Playground Link: link
Related Issues:
Activity
MartinJohns commentedon May 2, 2018
This is a known issue and is unfortunately "as intended", see #9235, #13161, #12287, and #7140. It's the same reason why accessing arrays by index does not result in a possibly undefined type, even tho theoretically the array could be empty. You'd need to add security checks basically at every indexer and array access, which would add a lot of bloat code to cases where usually you know beforehand that you have a value.
A common practice is to leave it as it is when you know that you will always access known keys, or explicitly add
undefinedto your indexer type:There's still an open proposal to add a compiler flag to automatically mark such indexers as
undefinedwith an additional strict-flag: #13778typescript-bot commentedon May 16, 2018
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.