Ts Template Literal Types

# TypeScript Template Literal Types

> Construct precise string types using template literal syntax and string manipulation types

## When to Use

- Creating type-safe string patterns (event names, CSS values, API routes)
- Generating getter/setter method names from property names
- Building type-safe routing or event systems
- Manipulating string literal types with Capitalize, Uppercase, Lowercase

## Instructions

1. **Basic template literal type:**

```typescript
type EventName = `on${Capitalize<'click' | 'hover' | 'focus'>}`;
// 'onClick' | 'onHover' | 'onFocus'
```

2. **Combine with union types** — distributes across all combinations:

```typescript
type Color = 'red' | 'blue' | 'green';
type Size = 'sm' | 'md' | 'lg';
type ClassName = `${Color}-${Size}`;
// 'red-sm' | 'red-md' | 'red-lg' | 'blue-sm' | 'blue-md' | ...
```

3. **Type-safe event emitters:**

```typescript
type Events = {
  click: { x: number; y: number };
  change: { value: string };
  submit: { data: FormData };
};

type OnEvent = `on${Capitalize<keyof Events>}`;
// 'onClick' | 'onChange' | 'onSubmit'
```

4. **String manipulation utility types:**

```typescript
type Upper = Uppercase<'hello'>; // 'HELLO'
type Lower = Lowercase<'HELLO'>; // 'hello'
type Cap = Capitalize<'hello'>; // 'Hello'
type Uncap = Uncapitalize<'Hello'>; // 'hello'
```

5. **Generate property names from a type:**

```typescript
type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};

type Setters<T> = {
  [K in keyof T as `set${Capitalize<string & K>}`]: (value: T[K]) => void;
};

type UserAccessors = Getters<{ name: string; age: number }>;
// { getName: () => string; getAge: () => number }
```

6. **Pattern matching with `infer`:**

```typescript
type ExtractRouteParams<T extends string> = T extends `${string}:${infer Param}/${infer Rest}`
  ? Param | ExtractRouteParams<`/${Rest}`>
  : T extends `${string}:${infer Param}`
    ? Param
    : never;

type Params = ExtractRouteParams<'/users/:userId/posts/:postId'>;
// 'userId' | 'postId'
```

7. **Type-safe CSS values:**

```typescript
type CSSUnit = 'px' | 'em' | 'rem' | '%' | 'vh' | 'vw';
type CSSLength = `${number}${CSSUnit}`;

function setWidth(width: CSSLength): void {
  /* ... */
}

setWidth('100px'); // OK
setWidth('2.5rem'); // OK
setWidth('100'); // Error: not assignable to CSSLength
```

8. **Dot-notation path types:**

```typescript
type DotPath<T, Prefix extends string = ''> = {
  [K in keyof T & string]: T[K] extends object ? DotPath<T[K], `${Prefix}${K}.`> : `${Prefix}${K}`;
}[keyof T & string];

type Paths = DotPath<{ user: { name: string; address: { city: string } } }>;
// 'user.name' | 'user.address.city'
```

## Details

Template literal types (TypeScript 4.1+) allow string types to be composed from other string literals, unions, and type-level transformations. They are the string equivalent of mapped types — they generate new types by iterating over string combinations.

**Distribution:** When a template literal contains union types, it produces the Cartesian product. `\`${A | B}-${C | D}\``produces`'A-C' | 'A-D' | 'B-C' | 'B-D'`.

**Intrinsic string manipulation types:** `Uppercase<S>`, `Lowercase<S>`, `Capitalize<S>`, `Uncapitalize<S>` are built-in compiler intrinsics — they work on any string literal type.

**Pattern matching:** Template literal types with `infer` can extract parts of string types. This enables parsing route parameters, CSS values, and other structured strings at the type level.

**Performance considerations:**

- Large Cartesian products slow the compiler. `\`${100 values}-${100 values}\`` creates 10,000 types
- Recursive template literal types have a depth limit. Keep recursion shallow
- Avoid template literals in hot type positions (deeply nested generics)

**Trade-offs:**

- Precise string typing catches typos at compile time — but generates verbose union types in IDE tooltips
- Route parameter extraction is elegant — but error messages when a route does not match are cryptic
- Template literals distribute over unions automatically — this is powerful but can produce unexpectedly large types

## Source

https://typescriptlang.org/docs/handbook/2/template-literal-types.html

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.