Description

Daniel Turan (Liero)

opened on May 16, 2016

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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'

Activity

1. mhegazy commented on May 16, 2016

   

   Mohamed Hegazy (mhegazy)

   on May 16, 2016

   Contributor

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   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.
2. Mohamed Hegazy (mhegazy)

   added

   QuestionAn issue which isn't directly actionable in codeAn issue which isn't directly actionable in code

   on May 16, 2016
3. Mohamed Hegazy (mhegazy)

   closed this as completedon May 16, 2016
4. Liero commented on May 17, 2016

   

   Daniel Turan (Liero)

   on May 17, 2016

   Author

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   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'.
5. microsoft

   locked and limited conversation to collaborators on Jun 19, 2018

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