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

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

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

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