Type definitions in TS + JS

options.ts

// the options array exported if you want to loop over the options
const options = ["a", "b", "c"] as const;

// the type which as no runtime overhead at all
type Option = typeof options[number]; // "a" | "b" | "c"

// the predicate for filtering or other runtime checks
const isOption = (option: string): option is Option =>
  (options).includes(option as Option)

// Parse, don’t validate
const getOption = (option: string): Option => {
  if (isOption(option)) return option;

  throw new Error(`Invalid option "${option}". Should be one of [${options.join(", ")}].`);
}

export { options, isOption, getOption }
export type { Option }