Pick / Omit from an array of field names

pick-from-array-of-fields.ts

type Foo = {
  bar: string;
  baz: string;
  qux: string;
};

// needs to be 'const' / immutable, otherwise ts doesn't know set of values
const includeFields = ['bar', 'baz'] as const;

// dynamic type to create a union of array values
//   ==> 'bar' | 'baz'
// - index of an array is type 'number', so this creates a type by collecting 
//   all values from object keys that are numbers, and for arrays, the index 
//   is the key.
type IncludedField = typeof includeFields[number];

// dynamic type with fields of 'Foo' who's keys are in `IncludedField'
//    ==> { bar: string, baz: string }
type FilteredFoo = Pick<Foo, IncludedField>

const typedPartial: FilteredFoo = {
  bar: 'bar value',
  baz: 'baz value',
  // @ts-expect-error
  qux: 'qux value',
};

// @ts-expect-error
typedPartial.qux === 'qux value';