IsFixedStringLiteralType

Problem Link

Problem

Determine if a type is a fixed (non-template) string literal type.

type T0 = IsFixedStringLiteralType<'foo'>          // true
type T1 = IsFixedStringLiteralType<string>         // false
type T2 = IsFixedStringLiteralType<`${string}`>    // false  (template literal = not fixed)
type T3 = IsFixedStringLiteralType<`foo`>          // true   (no interpolation = fixed)
type T4 = IsFixedStringLiteralType<'foo' | 'bar'>  // boolean (distributes)
type T5 = IsFixedStringLiteralType<number>         // false

Solution

type IsFixedStringLiteralType<T extends string> =
  string extends T
    ? false
    : T extends `${infer _Head}${infer _Tail}`
      ? _Head extends string
        ? _Tail extends ''
          ? true
          : IsFixedStringLiteralType<_Tail>
        : false
      : T extends ''
        ? true
        : false

A simpler approach:

type IsFixedStringLiteralType<T extends string> =
  [T] extends [never]
    ? false
    : string extends T
      ? false
      : T extends `${string & {}}${infer _}`
        ? false
        : true

Most concise approach:

type IsFixedStringLiteralType<T extends string> =
  string extends T ? false : `_${T}` extends `_${string & {}}` ? false : true

How it works:

1. string extends T — if T is the broad string type, return false.
2. Template literal types with interpolated string (like `${string}`) are not fixed literals.
3. A plain string literal like 'foo' is narrower than string, and concatenating a prefix _ doesn’t produce a template-literal-type — so the check distinguishes fixed from template literals.

Key Takeaways

• string extends T checks if T is the widened string type (not a literal).
• Template literal types with open interpolations (${string}) are structurally distinct from fixed literals.
• string & {} is a trick to get a non-widening string type for pattern matching purposes.