Interface with keys declared by string array

Question

I have an interface.

interface ICalculateFieldProps {
  fields: string[]
  set: string
  updateFunc: (valuesObj: object, oldValue: any) => any
}

I want valuesObj keys to be limited to fields: string[] items.

So for example, if pass ['width', 'height'] to fields. Then valuesObj must be

{
  height: any
  width: any
}

To add information to the question, please use the "edit" link on the question, rather than using comments. — T.J. Crowder
– T.J. Crowder, Commented Feb 8, 2020 at 12:01

T.J. Crowder · Accepted Answer · 2020-02-08 12:00:58Z

2

...if pass ['width', 'height'] to fields...

If you mean at runtime, you can't do this using TypeScript's type system. TypeScript does its work at compile time.

You can, of course, do it at runtime with runtime checks in the implementation of updateFunc:

// ...in a class implementing the interface...
updateFunc(valuesObj: object, oldValue: any): any {
    if (Reflect.ownKeys(valuesObj).some(key => !this.fields.includes(key))) {
        throw new Error(`Invalid property ${key} found in valuesObj.`);
    }
    // ...
}

...though it would probably be better if fields were a Set rather than an array.

answered Feb 8, 2020 at 12:00

T.J. Crowder

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1 Comment

Escobar Over a year ago

You are right about Set. I have a React component where I pass fields={['width', 'height']}. They are never dynamically calculated and I thought that I can check this restriction with generic or something like this.

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