Building the missing utilities in TypeScript

Existing Utilities#

In TypeScript, you have some built-in utilities, for instance Required transforms all the optional properties of a type or interface to non-optional properties. Here is an example from the doc:

interface Props {
  a?: number;
  b?: string;
}

const obj: Props = { a: 5 };

const obj2: Required<Props> = { a: 5 };
Property 'b' is missing in type '{ a: number; }' but required in type 'Required<Props>'.

but what if you want to only make property b required?

The one shot way#

What you want is to pick the property that you have to make non-optionnal, make it required then combine the result with the original interface or type and override the property that is now required. Here is the code:

interface Props {
  a?: number;
  b?: string;
}
interface PropsWithB = Props & Required<Pick<Props, 'b'>>

The advantage of Pick is that it can take multiple property keys, so we can make multiple properties required at once like this:

interface Props {
  a?: number;
  b?: string;
  c?: string;
}
interface PropsWithBandC = Props & Required<Pick<Props, 'b' | 'c'>>

here the interface PropsWithBandC is equivalent to this one

  interface Props {
  a?: number;
  b: string;
  c: string;
}

Building an utility#

Let's make this generic so that we can use it with any type and properties!

/**
 * Make fields required
 * Field names followed by a ? are required
 * (but can take any type/value declared originaly, including undefined or null)
 */
export type RequiredFields<T, K extends keyof T> = T & Required<Pick<T, K>>;

The utility is pretty straight forward if you know the syntax, you can pass an union for K so that you have multiple field required. Here is a usage example

interface Props {
  a?: number;
  b?: string;
  c?: string;
}
interface PropsWithBandC = RequiredFields<Props, 'b' | 'c'>

The resulting interface is equivalent to the one we've done in the one shot solution. You now have a simple utility with the ability to chose which properties you want to make required!