TypeScript shouldn't assume `{[name: string]: NonUndefinedType}` has no undefined properties

[KSXGitHub]

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:

[MartinJohns]

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 undefined to your indexer type:

interface Foo {
    [_: string]: number | undefined;
}

There's still an open proposal to add a compiler flag to automatically mark such indexers as undefined with an additional strict-flag: #13778

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.