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.
tsTry
typeWorld = "world";typeGreeting = `hello ${World }`;
When a union is used in the interpolated position, the type is the set of every possible string literal that could be represented by each union member:
tsTry
typeEmailLocaleIDs = "welcome_email" | "email_heading";typeFooterLocaleIDs = "footer_title" | "footer_sendoff";typeAllLocaleIDs = `${EmailLocaleIDs |FooterLocaleIDs }_id`;
For each interpolated position in the template literal, the unions are cross multiplied:
tsTry
typeAllLocaleIDs = `${EmailLocaleIDs |FooterLocaleIDs }_id`;typeLang = "en" | "ja" | "pt";typeLocaleMessageIDs = `${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:
tsTry
constpassedObject = {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, sincefirstName
is typed asstring
, the callback for thefirstNameChanged
event expects astring
to be passed to it at call time. Similarly events associated withage
should expect to be called with anumber
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.
tsTry
constperson =makeWatchedObject ({firstName : "Saoirse",lastName : "Ronan",age : 26,});// makeWatchedObject has added `on` to the anonymous Objectperson .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:
tsTry
typePropEventSource <Type > = {on (eventName : `${string & keyofType }Changed`,callback : (newValue : any) => void): void;};/// Create a "watched object" with an `on` method/// so that you can watch for changes to properties.declare functionmakeWatchedObject <Type >(obj :Type ):Type &PropEventSource <Type >;
With this, we can build something that errors when given the wrong property:
tsTry
constperson =makeWatchedObject ({firstName : "Saoirse",lastName : "Ronan",age : 26});person .on ("firstNameChanged", () => {});// Prevent easy human error (using the key instead of the event name)Argument of type '"firstName"' is not assignable to parameter of type '"firstNameChanged" | "lastNameChanged" | "ageChanged"'.2345Argument of type '"firstName"' is not assignable to parameter of type '"firstNameChanged" | "lastNameChanged" | "ageChanged"'.person .on ("firstName" , () => {});// It's typo-resistantArgument of type '"frstNameChanged"' is not assignable to parameter of type '"firstNameChanged" | "lastNameChanged" | "ageChanged"'.2345Argument of type '"frstNameChanged"' is not assignable to parameter of type '"firstNameChanged" | "lastNameChanged" | "ageChanged"'.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:
- The literal used in the first argument is captured as a literal type
- That literal type can be validated as being in the union of valid attributes in the generic
- The type of the validated attribute can be looked up in the generic’s structure using Indexed Access
- This typing information can then be applied to ensure the argument to the callback function is of the same type
tsTry
typePropEventSource <Type > = {on <Key extends string & keyofType >(eventName : `${Key }Changed`,callback : (newValue :Type [Key ]) => void): void;};declare functionmakeWatchedObject <Type >(obj :Type ):Type &PropEventSource <Type >;constperson =makeWatchedObject ({firstName : "Saoirse",lastName : "Ronan",age : 26});person .on ("firstNameChanged",newName => {console .log (`new name is ${newName .toUpperCase ()}`);});person .on ("ageChanged",newAge => {if (newAge < 0) {console .warn ("warning! negative age");}})
Here we made on
into a generic method.
When a user calls with the string "firstNameChanged"
, TypeScript will try to infer the right type for Key
.
To do that, it will match Key
against the content before "Changed"
and infer the string "firstName"
.
Once TypeScript figures that out, the on
method can fetch the type of firstName
on the original object, which is string
in this case.
Similarly, when called with "ageChanged"
, TypeScript finds the type for the property age
which is number
.
Inference can be combined in different ways, often to deconstruct strings, and reconstruct them in different ways.
Intrinsic String Manipulation Types
To help with string manipulation, TypeScript includes a set of types which can be used in string manipulation. These types come built-in to the compiler for performance and can’t be found in the .d.ts
files included with TypeScript.
Uppercase<StringType>
Converts each character in the string to the uppercase version.
Example
tsTry
typeGreeting = "Hello, world"typeShoutyGreeting =Uppercase <Greeting >typeASCIICacheKey <Str extends string> = `ID-${Uppercase <Str >}`typeMainID =ASCIICacheKey <"my_app">
Lowercase<StringType>
Converts each character in the string to the lowercase equivalent.
Example
tsTry
typeGreeting = "Hello, world"typeQuietGreeting =Lowercase <Greeting >typeASCIICacheKey <Str extends string> = `id-${Lowercase <Str >}`typeMainID =ASCIICacheKey <"MY_APP">
Capitalize<StringType>
Converts the first character in the string to an uppercase equivalent.
Example
tsTry
typeLowercaseGreeting = "hello, world";typeGreeting =Capitalize <LowercaseGreeting >;
Uncapitalize<StringType>
Converts the first character in the string to a lowercase equivalent.
Example
tsTry
typeUppercaseGreeting = "HELLO WORLD";typeUncomfortableGreeting =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;
}