javascript - Typescript dynamically create interface

Question

I use simple-schema to define DB schemas in an object:

{
   name: 'string',
   age: 'integer',
   ...
}

Is it somehow possible to create an interface or class from this object, so I don't have to type everything twice?

Answer 1

You can do this, but it might be more trouble than it's worth unless you think you might be changing the schema. TypeScript doesn't have built-in ways of inferring types in a way that you want, so you have to coax and cajole it to do so:

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First, define a way of mapping the literal names 'string' and 'integer' to the TypeScript types they represent (presumably string and number respectively):

type MapSchemaTypes = {
  string: string;
  integer: number;
  // others?
}

type MapSchema<T extends Record<string, keyof MapSchemaTypes>> = {
  -readonly [K in keyof T]: MapSchemaTypes[T[K]]
}

Now if you can take an appropriately typed schema object like the one you specified, and get the associated type from it:

const personSchema = {name: 'string', age: 'integer'}; 
type Person = MapSchema<typeof personSchema>; // ERROR

Oops, the problem is that personSchema is being inferred as {name: string; age: string} instead of the desired {name: 'string'; age: 'integer'}. You can fix that with a type annotation:

const personSchema: { name: 'string', age: 'integer' } = { name: 'string', age: 'integer' }; 
type Person = MapSchema<typeof personSchema>; // {name: string; age: number};

But now it feels like you're repeating yourself. Luckily there is a way to force it to infer the proper type:

function asSchema<T extends Record<string, keyof MapSchemaTypes>>(t: T): T {
  return t;
}
const personSchema = asSchema({ name: 'string', age: 'integer' }); // right type now
type Person = MapSchema<typeof personSchema>; // {name: string; age: number};

UPDATE 2020-06: in more recent TS versions you can use a const assertion to get the same result:

const personSchema = { name: 'string', age: 'integer' } as const;
type Person = MapSchema<typeof personSchema>;

That works!

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See it in action on the Typescript Playground. Hope that helps; good luck!