Validate indexed properties

[Liero]

When object is annotated with following definition:

interface Person {
  "Id": number
  "First Name": string,
  "Last Name": string,
}

I would like to access the "First Name" and "Last Name" properties in type safe manner.

var person : Person = { Id: 5, "First Name": "Lorem", "Last Name": "Ipsum" };
var firstName = person["First Name"]; //this is neither validated by compiler nor I get auto completion for the properties
var _undefined = person["undefined"]; //compiler does not complain.

similarly:

person["First Name"] = "Lorem"; //this is neither validated by compiler nor I get auto completion for the properties
person["undefined"] = "Ipsum"; //compiler does not complain.

I have tried experimenting with restricting index signature type:

interface Person {
  [key: "First Name" | "Last Name"]: string;
} 

however, it compiler complains:

An index signature parameter type must be 'string' or 'number'

[mhegazy]

The compiler does know that values of type Person have a property called First Name and that its type is string. see:

It also knows that all JavaScript objects can be indexed in using strings. For these it does not know the type of the property, so it gives it a type any.

You can use --noImplicitAny flag to catch these cases and flag them as errors.

As for completion for the name, this will be supported by #8428.

[Liero]

ahh, ok. I thought --noImplicitAnyis related to unannotated variables and parameters, but person is annotated. Hmm, I would welcome if those two were splitted into two flags (e.g --noImplicitAny, --noImplicitIndex), but that's probably not gonna happen. Also the error message could be: "Property 'undefined' does not exists on type 'Person'.