When you are developing software in TypeScript, you may come across scenarios where you need to declare the same entity multiple times. This can happen when you are working with external libraries or when you want to extend an existing type. In such cases, TypeScript provides a feature called "Declaration Merging" that allows you to merge multiple declarations of the same entity into a single definition.
In this post, we will explore the concept of Declaration Merging in TypeScript and learn how to use it effectively.
Declaration Merging is a feature in TypeScript that allows you to combine multiple declarations of the same entity into a single definition. This feature is useful when you are working with external libraries that have their own type definitions or when you want to extend an existing type.
Declaration Merging works by aggregating the declarations of the same entity and creating a single definition that includes all the properties and methods from each declaration. This can be done for interfaces, classes, functions, and namespaces.
To use Declaration Merging in TypeScript, you need to follow a few rules:
Let's look at some examples to understand how to use Declaration Merging.
Suppose you have two interfaces with the same name:
interface Person {
name: string;
}
interface Person {
age: number;
}
In this case, TypeScript will automatically merge the two interfaces into a single definition:
interface Person {
name: string;
age: number;
}
Now, you can use the Person
interface with both name
and age
properties.
Suppose you have two functions with the same name:
function greet(name: string): void {
console.log(`Hello, ${name}!`);
}
function greet(message: number): void {
console.log(`Your message has ${message} characters.`);
}
In this case, TypeScript will automatically merge the two functions into a single definition:
function greet(name: string): void;
function greet(message: number): void;
function greet(nameOrMessage: string | number): void {
if (typeof nameOrMessage === 'string') {
console.log(`Hello, ${nameOrMessage}!`);
} else {
console.log(`Your message has ${nameOrMessage} characters.`);
}
}
Now, you can call the greet
function with either a string or a number argument.
Suppose you have two namespaces with the same name:
namespace MyNamespace {
export const name = 'John';
}
namespace MyNamespace {
export const age = 30;
}
In this case, TypeScript will automatically merge the two namespaces into a single definition:
namespace MyNamespace {
export const name = 'John';
export const age = 30;
}
Now, you can access both name
and age
properties of the MyNamespace
namespace.
Here are some additional things to keep in mind when working with Declaration Merging:
declare
keyword to tell TypeScript that a declaration is external and will be provided at runtime. This is useful when working with libraries that have their own type definitions.namespace
keyword to group related declarations together. This is useful when you have a large number of declarations and want to organize them into logical groups.Declaration Merging is a powerful feature in TypeScript that allows you to combine multiple declarations of the same entity into a single definition. This feature is useful when you are working with external libraries or when you want to extend an existing type.
By following the rules and guidelines outlined in this post, you can use Declaration Merging effectively and improve the quality of your TypeScript code.