Template Literal Types

Template literal types build on string literal types, and have the ability to expand into many strings via unions.

They have the same syntax as template literal strings in JavaScript, but are used in type positions. When used with concrete literal types, a template literal produces a new string literal type by concatenating the contents.

type World = "world";

type Greeting = `hello ${World}`;
//   ^?

When a union is used in the interpolated position, the type is the set of every possible string literal that could be represented by each union member:

type EmailLocaleIDs = "welcome_email" | "email_heading";
type FooterLocaleIDs = "footer_title" | "footer_sendoff";

type AllLocaleIDs = `${EmailLocaleIDs | FooterLocaleIDs}_id`;
//   ^?

For each interpolated position in the template literal, the unions are cross multiplied:

type EmailLocaleIDs = "welcome_email" | "email_heading";
type FooterLocaleIDs = "footer_title" | "footer_sendoff";
// ---cut---
type AllLocaleIDs = `${EmailLocaleIDs | FooterLocaleIDs}_id`;
type Lang = "en" | "ja" | "pt";

type LocaleMessageIDs = `${Lang}_${AllLocaleIDs}`;
//   ^?

We generally recommend that people use ahead-of-time generation for large string unions, but this is useful in smaller cases.

String Unions in Types

The power in template literals comes when defining a new string based on information inside a type.

Consider the case where a function (makeWatchedObject) adds a new function called on() to a passed object. In JavaScript, its call might look like: makeWatchedObject(baseObject). We can imagine the base object as looking like:

// @noErrors
const passedObject = {
  firstName: "Saoirse",
  lastName: "Ronan",
  age: 26,
};

The on function that will be added to the base object expects two arguments, an eventName (a string) and a callback (a function).

The eventName should be of the form attributeInThePassedObject + "Changed"; thus, firstNameChanged as derived from the attribute firstName in the base object.

The callback function, when called:

• Should be passed a value of the type associated with the name attributeInThePassedObject; thus, since firstName is typed as string, the callback for the firstNameChanged event expects a string to be passed to it at call time. Similarly events associated with age should expect to be called with a number argument
• Should have void return type (for simplicity of demonstration)

The naive function signature of on() might thus be: on(eventName: string, callback: (newValue: any) => void). However, in the preceding description, we identified important type constraints that we'd like to document in our code. Template Literal types let us bring these constraints into our code.

// @noErrors
declare function makeWatchedObject(obj: any): any;
// ---cut---
const person = makeWatchedObject({
  firstName: "Saoirse",
  lastName: "Ronan",
  age: 26,
});

// makeWatchedObject has added `on` to the anonymous Object

person.on("firstNameChanged", (newValue) => {
  console.log(`firstName was changed to ${newValue}!`);
});

Notice that on listens on the event "firstNameChanged", not just "firstName". Our naive specification of on() could be made more robust if we were to ensure that the set of eligible event names was constrained by the union of attribute names in the watched object with "Changed" added at the end. While we are comfortable with doing such a calculation in JavaScript i.e. Object.keys(passedObject).map(x => `${x}Changed`), template literals inside the type system provide a similar approach to string manipulation:

type PropEventSource<Type> = {
  on(
    eventName: `${string & keyof Type}Changed`,
    callback: (newValue: any) => void
  ): void;
};

/// Create a "watched object" with an `on` method
/// so that you can watch for changes to properties.
declare function makeWatchedObject<Type>(
  obj: Type
): Type & PropEventSource<Type>;

With this, we can build something that errors when given the wrong property:

// @errors: 2345
type PropEventSource<Type> = {
  on(
    eventName: `${string & keyof Type}Changed`,
    callback: (newValue: any) => void
  ): void;
};

declare function makeWatchedObject<T>(obj: T): T & PropEventSource<T>;
// ---cut---
const person = makeWatchedObject({
  firstName: "Saoirse",
  lastName: "Ronan",
  age: 26,
});

person.on("firstNameChanged", () => {});

// Prevent easy human error (using the key instead of the event name)
person.on("firstName", () => {});

// It's typo-resistant
person.on("frstNameChanged", () => {});

Inference with Template Literals

Notice that we did not benefit from all the information provided in the original passed object. Given change of a firstName (i.e. a firstNameChanged event), we should expect that the callback will receive an argument of type string. Similarly, the callback for a change to age should receive a number argument. We're naively using any to type the callback's argument. Again, template literal types make it possible to ensure an attribute's data type will be the same type as that attribute's callback's first argument.

The key insight that makes this possible is this: we can use a function with a generic such that:

1. The literal used in the first argument is captured as a literal type
2. That literal type can be validated as being in the union of valid attributes in the generic
3. The type of the validated attribute can be looked up in the generic's structure using Indexed Access
4. This typing information can then be applied to ensure the argument to the callback function is of the same type

type PropEventSource<Type> = {
  on<Key extends string & keyof Type>(
    eventName: `${Key}Changed`,
    callback: (newValue: Type[Key]) => void
  ): void;
};

declare function makeWatchedObject<Type>(
  obj: Type
): Type & PropEventSource<Type>;

const person = makeWatchedObject({
  firstName: "Saoirse",
  lastName: "Ronan",
  age: 26,
});

person.on("firstNameChanged", (newName) => {
  //                        ^?
  console.log(`new name is ${newName.toUpperCase()}`);
});

person.on("ageChanged", (newAge) => {
  //                  ^?
  if (newAge < 0) {
    console.warn("warning! negative age");
  }
});

Here we made on into a generic method.

When a user calls with the string "firstNameChanged", TypeScript will try to infer the right type for Key. To do that, it will match Key against the content before "Changed" and infer the string "firstName". Once TypeScript figures that out, the on method can fetch the type of firstName on the original object, which is string in this case. Similarly, when called with "ageChanged", TypeScript finds the type for the property age which is number.

Inference can be combined in different ways, often to deconstruct strings, and reconstruct them in different ways.

Intrinsic String Manipulation Types

To help with string manipulation, TypeScript includes a set of types which can be used in string manipulation. These types come built-in to the compiler for performance and can't be found in the .d.ts files included with TypeScript.

Uppercase<StringType>

Converts each character in the string to the uppercase version.

Example

type Greeting = "Hello, world";
type ShoutyGreeting = Uppercase<Greeting>;
//   ^?

type ASCIICacheKey<Str extends string> = `ID-${Uppercase<Str>}`;
type MainID = ASCIICacheKey<"my_app">;
//   ^?

Lowercase<StringType>

Converts each character in the string to the lowercase equivalent.

Example

type Greeting = "Hello, world";
type QuietGreeting = Lowercase<Greeting>;
//   ^?

type ASCIICacheKey<Str extends string> = `id-${Lowercase<Str>}`;
type MainID = ASCIICacheKey<"MY_APP">;
//   ^?

Capitalize<StringType>

Converts the first character in the string to an uppercase equivalent.

Example

type LowercaseGreeting = "hello, world";
type Greeting = Capitalize<LowercaseGreeting>;
//   ^?

Uncapitalize<StringType>

Converts the first character in the string to a lowercase equivalent.

Example

type UppercaseGreeting = "HELLO WORLD";
type UncomfortableGreeting = Uncapitalize<UppercaseGreeting>;
//   ^?

Technical details on the intrinsic string manipulation types

The code, as of TypeScript 4.1, for these intrinsic functions uses the JavaScript string runtime functions directly for manipulation and are not locale aware.

function applyStringMapping(symbol: Symbol, str: string) {
    switch (intrinsicTypeKinds.get(symbol.escapedName as string)) {
        case IntrinsicTypeKind.Uppercase: return str.toUpperCase();
        case IntrinsicTypeKind.Lowercase: return str.toLowerCase();
        case IntrinsicTypeKind.Capitalize: return str.charAt(0).toUpperCase() + str.slice(1);
        case IntrinsicTypeKind.Uncapitalize: return str.charAt(0).toLowerCase() + str.slice(1);
    }
    return str;
}