Describing an Interface where unknown properties are of a specific type

[amarzavery]

• This is the json schema definition or a model defined in the OpenAPI specification 2.0.

{
  "Foo": {
    "properties": {
      "name": {
        "type": "string"
      },
      "age": {
        "type": "number"
      }
    },
    "additionalProperties": {
      "type": "string"
    }
  }
}

We would like to model an Interface in TS for the above json schema definition as follows:

interface Foo {
  name: string;
  age: number;
  [prop: string]: string;
}

However, TS gives the following error:

Property 'age' of type 'number' is not assignable to string index type 'string'.

• Well, we can make the value of the indexed property a loose type any

interface Foo {
  name: string;
  age: number;
  [prop: string]: any; // but this defeats the purpose of providing good typing.
}

The above approach would be perfectly fine for "additionalProperties": true or "additionalProperties": { "type": "object" }, but not for specific types.

• Another approach would be to make additionalProperties a Dictionary<string>.

interface Foo {
  name: string;
  age: number;
  additionalProperties: { [prop: string]: string}; 
}

This would be convenient for C#, Python, etc. developers (as they deal with objects in that language). However, it would be inconvenient for JS developers to provide (or access) a JSON object that has unknown properties in a property bag called additionalProperties.

Any help/idea in solving this problem will be very helpful.

[alexrock]

Intersection types allow this

type Foo =
  {
    name: string;
    age: number;
  } & {
    [prop: string]: string;
  };

[amarzavery]

@alexrock - Thanks for letting me know. Learnt something new today.

I tried the following

type Foo = {
  name: string;
  age: number;
} & {
  [prop: string]: string;
};

var f: Foo = {
  name: "foo",
  age: 10,
  color: "red"
};

and it still gives an error:

Type '{ name: string; age: number; color: string; }' is not assignable to type 'Foo'.
  Type '{ name: string; age: number; color: string; }' is not assignable to type '{ [prop: string]: string; }'.
    Property 'age' is incompatible with index signature.
      Type 'number' is not assignable to type 'string'.

NOTE: I also found some info at @basarat's TS gitbook which explains why all members must conform to the index signature.

[alexrock]

@amarzavery - I understand. Object literals don't allow this, but member access is still allowed and correctly typed.

type Foo =
  {
    name: string
    age: number
  } & {
    [prop: string]: string
  }

let foo: Foo = JSON.parse(`{
  "name": "foo",
  "age": 10,
  "color": "red"
}`)

foo.color = 0x0000ff // type error
foo.color = 'blue'   // ok, color is string

I can't say, absolutely, if this is the intended behavior.

[DanielRosenwasser]

Looks like a potential duplicate of #17867 (partially discussed here #18394 (comment)).

[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.