What are template literal types in Typescript ?

Template literal types in TypeScript allow the construction of new string literal types by combining existing string literal types using template literal syntax.

• They enable the creation of complex string patterns by embedding unions and other literal types within template literals.
• This feature enhances type safety by allowing developers to define and enforce specific string formats at the type level.

type Size = "small" | "medium" | "large";
type SizeMessage = `The selected size is ${Size}.`;

let message: SizeMessage;

message = "The selected size is small.";  // Valid
message = "The selected size is extra-large.";  // Error

• Size is a union type representing possible sizes.
• SizeMessage is a template literal type that constructs specific string patterns incorporating each Size value.
• The variable message can only be assigned strings that match the SizeMessage pattern.

Output:

Type '"The selected size is extra-large."' is not assignable to type 'SizeMessage'.

More Example of template literal types in Typescript

Defining Paths Using TypeScript Literals

type ApiEndpoints = "users" | "posts" | "comments";
type ApiPath = `/api/${ApiEndpoints}`;

const userPath: ApiPath = "/api/users";
const invalidPath: ApiPath = "/api/unknown";

• ApiEndpoints is a union type representing possible API endpoint names.
• ApiPath is a template literal type that dynamically constructs string patterns prefixed with /api/ followed by one of the ApiEndpoints.
• userPath is valid because it matches the constructed pattern, while invalidPath throws an error.

Output:

Type '"/api/unknown"' is not assignable to type 'ApiPath'.

Formatting Messages Using Template Literals

type Status = "success" | "error" | "loading";
type StatusMessage = `The operation is ${Status}.`;

const successMessage: StatusMessage = "The operation is success.";
const invalidMessage: StatusMessage = "The operation is pending.";

• Status is a union type representing possible operation statuses.
• StatusMessage constructs string patterns to describe the status of an operation.
• successMessage is valid because it matches the pattern, but invalidMessage throws an error as "pending" is not part of Status.

Output:

Type '"The operation is pending."' is not assignable to type 'StatusMessage'.