TypeScript: Check if an Array Constant Includes a Value

The Problem

When an array is marked as a const (readonly), its type becomes a union of its values.

const VALID_VALUES = ['a', 'b'] as const;

const isValid = (value: string): boolean => (
  // ❌ Argument of type 'string' is not assignable to parameter of type '"a" | "b"'
  VALID_VALUES.includes(value);
)

Solution 1 - Widen the type

Explicitly set the type to account for other possible values.

Caveat: Widens the type across the board.

const VALID_VALUES: Readonly<string[]> = ['a', 'b'] as const;

const isValid = (value: string): boolean => (
  // ✅
  VALID_VALUES.includes(value);
)

Solution 2 - Assert the type during the check

Cast the constant during the check.

const VALID_VALUES = ['a', 'b'] as const;

const isValid = (value: string): boolean => (
  // ✅
  (VALID_VALUES as Readonly<string[]>).includes(value);
)