Enforcing types in Typescript interface

Question

I have a interface for typing that looks like the following

interface Bob{
    a: number;
    b: number;
    c: number;
}

However, I don't really want number as any number can be valid. How can I enforce the type of Bob to be numbers strictly in a JSON file like the following.

{
  "None": -1,
  "foo": 22,
  "moo": 55,
  "raar": 13,
  "zaar": 2,
  "bar": 0,
  "fooboo": 22,
  "mooboo": 1000,
}

That JSON contains around 200 entries hence doing a manual typedef is not optimal.

What I aim to achieve

const myvar: Bob;
myvar.a = 2222 // this should throw an error as this number is not defined in the JSON
myvar.b = -1 //this is ok as it is in the JSON

Thanks!

Import json (it will be typed), then do type values = typeof v[keyof typeof v]; (where v is your imported json) — zerkms
– zerkms, Commented Jan 17, 2021 at 23:02
wont work. You still can user other numbers -> typescriptlang.org/play?#code/… — Martin Godzina
– Martin Godzina, Commented Jan 17, 2021 at 23:16
@MartinGodzina will work, if you type it as const typescriptlang.org/play?#code/… — zerkms
– zerkms, Commented Jan 17, 2021 at 23:20
Thank you @zerkms! Works like a charm. Do you mind explaining what does the as const do at the end? — bobob
– bobob, Commented Jan 17, 2021 at 23:33

bobob · Accepted Answer · 2021-01-17 23:40:30Z

3

Thanks to @zerkms for the answer.

const json = {
  "None": -1,
  "foo": 22,
  "moo": 55,
  "raar": 13,
  "zaar": 2,
  "bar": 0,
  "fooboo": 22,
  "mooboo": 1000,
} as const;

type $Values<T extends object> = T[keyof T];
type supportedNumbers = typeof json[keyof typeof json];

interface Bob{
    a: supportedNumbers;
    b: supportedNumbers;
    c: supportedNumbers;
}

const NiceBob: Bob = {
  a: 3909, //error
  b: 319039, //error
  c: -1 //no error
};

Example can be found here

Docs can be found here

answered Jan 17, 2021 at 23:40

bobob

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3 Comments

Gerrit Begher Over a year ago

Do you need to refer to the const json at runtime? Or do you only define it to define the type supportedNumbers? If it is the latter: You could go without having a const by using type json = { ... } and type supportedNumbers = json[keyof json].

Gerrit Begher Over a year ago

typescriptlang.org/play?#code/…

bobob Over a year ago

in my case const is good since it is loaded when the app is running and will not be modified

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