[Typescript] Tips: Throw detailed error messages for type checks

You can throw detailed error messages for type checks. Here, I move a runtime check in a function to the type level, meaning you get a detailed error if you use it wrong.

 

const deepEqualCompare = <T extends any>(
  a: T, b: T
): boolean | Error => {
  if (Array.isArray(a) || Array.isArray(b)) {
    throw new Error("You cannot compare two arrays using deepEqualCompare")
  }
  return a === b
}

deepEqualCompare(1, 1)
deepEqualCompare([], ["a"])

 

Using type check:

export type checkForBadArgs<Arg> = Arg extends any[]
  ? "You cannot compare two arrays using deepEqualCompare"
  : Arg;

const deepEqualCompare = <T extends any>(
  a: checkForBadArgs<T>, b: checkForBadArgs<T>
): boolean | Error => {
  if (Array.isArray(a) || Array.isArray(b)) {
    throw new Error("You cannot compare two arrays using deepEqualCompare")
  }
  return a === b
}

deepEqualCompare(1, 1)
const res = deepEqualCompare([], ["a"]) // type error