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With this library, you can create 'proxy references' for your objects and access many methods that actually doesn't exists in them! This is pretty useful in the following scenarios:

  • where you need to create a method that returns an object with a lot of methods, but the code that'll use that result will only access a few of them;
  • When you have a base class and you want to implement functionalities in it without changing the original contract;

Depending on the number of methods you'll proxy through extension-methods, you can achieve a 99% faster operation than a simple new Class()

How to use it

First, you need to obtain your extension object, like this:

import { getExtender } from 'extension-methods';

export interface MyObjectExtended {
  concatFoo(): MyObjectExtended;
  concatBar(): MyObjectExtended;

export interface MyObject {
  value: string;

export const myObjectExtension = getExtender({
    concatFoo(this: MyObject) {
      return extend({ value: `${this.value}_foo` }, myObjectExtension);
    concatBar(this: MyObject) {
      return extend({ value: `${this.value}_bar` }, myObjectExtension);

Now, take a look in the extend function called in the above code: this is the one who applies the extension! The first parameter must be the definition of this, as it will be a reference to the object being extended, so, you can access it and do whatever you want! So, when you need to extend methods in some object, just do it like this:

import { extend } from 'extension-methods';
import { myObjectExtension, MyObject, MyObjectExtended } from './my-object-extension';
const extended: MyObjectExtended = extend({ value: 'my string' }, myObjectExtension);

// Result:
// my string_foo
// my string_bar
// my string_foo_bar

Look that does methods actually are not present in the extended const (or in the original object), but extension-methods make it be accessible at runtime, making the call to extend being much faster! Also, in the Extender implementation, the return of concatFoo and concatBar also applies the extend function to the result, which creates a fluent interface for this use, resulting in the chained  call as you can see above! All of that, with less overload possible!

Extending Classes

You can also create extended version of Classes that will create instances of such classes with the exactly same benefits of an extended object. To achieve this, just use the method extendClass:

import * as examplePackage from './example-class';
import { extendClass, getExtender } from 'extension-methods';

declare module './example-class.spec' {
  interface ExampleClass {
    method1(): number;

const extender = getExtender({
  method1(this: ExampleClass) {
    return this.someProperty * 3;

export const ExtendedClass = extendClass(ExampleClass, extender);

Notice that an interface with the same name of the class is declared in the same module from where the class is imported. Doing this will make the new methods visible anywhere this code is imported.

Now, import your ExtendedClass where you want to use it!

import { ExtendedClass } './extended-class';

const test = new ExtendedClass();


And that's it, it'll just work!


  • If some method exists in the original object and also is declared in the Extender, the original method will be used;
  • extension-methods can't be used with primitive values like stringnumber and boolean;
  • extend will naturally returns a type that is a join between the real object and the extension methods declared, but it is recommendable, if you want a cleaner type or to return such value as a result of a function, to create an interface that represent it, as you can see in the examples above;

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