TypeScript merge Function interface, extend Function prototype

Question

interface Function {
    next(next: Function): Function;
    prev(prev: Function): Function;
}

Function.prototype.next = function(next) {
    const prev = this;
    return function() {
        return next.call(this, prev.apply(this, arguments));
    };
};

Function.prototype.prev = function(prev) {
    const next = this;
    return function() {
        return next.call(this, prev.apply(this, arguments));
    };
};

const f1 = function() { console.log("f1"); };
const f2 = () => console.log("f2");
const f3 = new Function("console.log('f3');");

f1.next(f2).next(f3)();

I wanted to do bad thing and extend Function prototype in TypeScript compiling to ES6. While this code works well in TypeScript Playground it fails in tsc 1.8.10 (Property <<name>> does not exist on type 'Function') because it can't merge with Function definition in lib.es6.d.ts .

Any ideas how to do it properly?

Answer 1

According to the docs :

Similarly, the global scope can be augmented from modules using a declare global declaration.

Note the wording from modules . In other words, put the augmentations in a different module, and import it, which is when the merging occurs. Also, put the new prototype definitions in the same file.

// augment.ts
export {};

declare global {
  interface Function {
    next(next: Function): Function;
    prev(prev: Function): Function;
  }
}
Function.prototype.next = function(next) {
  const prev = this;
  return function() {
    return next.call(this, prev.apply(this, arguments));
  };
};
Function.prototype.prev = function(prev) {
  const next = this;
  return function() {
    return next.call(this, prev.apply(this, arguments));
  };
};


// test.ts
import './augment';

const f1 = function() { console.log("f1"); };
const f2 = () => console.log("f2");
const f3 = new Function("console.log('f3');");

f1.next(f2).next(f3)();

Output:

f1
f2
f3