Add extract function

[aleclarson]

A type-safe utility for extracting a value from a set of options. Returns undefined if the input value isn't in the set. Designed to work elegantly with the nullish coalescing operator (??) for setting default values.

Examples

// Environment handling
extract(process.env.NODE_ENV, ['production']) ?? 'development'

// Feature flags based on deployment
extract(pulumi.getStack(), ['prod', 'staging']) ?? 'local'

// User roles
extract(user.role, ['admin', 'moderator']) ?? 'user'

Implementation

type IfExists<T, U> = T extends never ? never : U

function extract<TInput, const TOutput>(
  input: TInput,
  outputs: TOutput[],
): Extract<TOutput, TInput> | IfExists<Exclude<TInput, TOutput>, undefined> {
  const index = outputs.indexOf(input as any)
  return (index === -1 ? undefined : outputs[index]) as any
}

The function leverages TypeScript's type system to ensure type safety when working with literal types and unions. The return type handles cases where TInput is either fully contained within or partially overlaps with TOutput.