Generics and Known Fields

If you want to use a generic and refer to its fields, use this.

<T, K extends keyof T>(field: K) =>

• T is your type
• keyof T - union of the keys of T
• K extends - makes that list of keys a type

Now you can also specify what field is, the K (or key) type.

Example

const sortBy = <T, K extends keyof T>(field: K) =>
  (a: T, b: T): number => {
    const aValue = a[field]
    const bValue = b[field]
    if (aValue < bValue) {
      return -1
    } if (aValue < bValue) {
      return 1
    }
    return 0
  }

type Measurement = {
  degrees: number,
}

const byDegrees = sortBy<Measurement, 'degrees'>('degrees')