I really like C#'s extension method feature. I was trying to experiment it on Typescript with decorators.
export function extension(ctr: any ) {
let originalFunction: Function;
return function (target, propertyKey: string, descriptor: PropertyDescriptor) {
originalFunction = descriptor.value;
ctr.prototype[propertyKey] = function(...args){
return originalFunction(this, ...args);
}
}
}I created a decorator factory named
extensionctrSince Typescript doesn't give a build in extension method concept to us, as a work around, we are adding the the function to the prototype of the passed in class.
export class User {
constructor(
public name: string = 'joe',
public lastname: string = 'doe'
){}
}
export class Extensions {
@extension(User)
static test(user: User){
console.log(user.name , 'this works');
}
@extension(User)
static gaveMoney(thisArg: User, amount: number, to: User){
console.log(`${thisArg.name} gave ${to.name} \$${amount}`);
}
}
const user = new User();
const user2 = new User('mehmet', '');
user.gaveMoney(10, user2);
user2.gaveMoney(20, user);
user.test();
As you can see that the methods that we decorated with
extensionUserIf you are getting an error
Property 'gaveMoney' does not exist on type 'User'(user as any).gaveMoney(10, user2);or
export interface User{
test; // adding the name only will fix the error
gaveMoney(n: number, u:User) // this will give intellisense
}Problem
By mutating the prototype essentially we are polluting the prototype. So this trick will not work on
frozensealedUser2
export class Extensions2 {
@extension(User2)
static test(user: User2){
console.log(user.name , 'this works');
}
}
export class User2 {
name = 'john'
}
Object.freeze(User2);
Object.freeze(User2.prototype);
const sealedUser = new User2();
sealedUser.test();
Conclusion
Although this trick will work for our stuff, probably it won't work on most third party libraries assuming that they are safeguarded against prototype pollution. The beauty of extension methods is that it should work on classes that we don't have any control over.
Let's hope that Typescript team decides to add this awesome feature. (https://github.com/microsoft/TypeScript/issues/9)
Demo for this article: https://stackblitz.com/edit/ts-ms-extension-methods-experiment?file=index.ts
