Interface with optional properties is not assignable to indexable type with strict-null-checks

[maghis]

I understand this is more a feature request then a bug report :)

TypeScript Version: 2.3.2

Code

const test: { [key: string]: string } = {} as { foo?: string };

Expected behavior:
A type with optional properties should be assignable to an indexable, even if the indexable values do not include undefined.

Actual behavior:

error TS2322: Type '{ foo?: string | undefined; }' is not assignable to type '{ [key: string]: string; }'.
  Property 'foo' is incompatible with index signature.
    Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'.

[mhegazy]

the type should be const test: { [key: string]: string | undefined } = {} as { foo?: string };.

see #13778 for relevant discussion on why the indexer does not include undefined by default.

[maghis]

@mhegazy thanks for the pointer, the for loop on an array use case made it perfectly clear.

Should I close this?