How to spy on declared functions with sinon without passing extra arguments

[GJZwiers]

I have a function A that calls another function B:

function A() {
  B();
}

function B() {

}

And I want to write a test that wraps function B using sinon and makes some assertions, such as if B has been called after invoking A:

const spy = sinon.spy(B);

// do test, call A somewhere

assertEquals(spy.called, true);

The only way this works is if I add a parameter to A so I can replace B with the spy function for the test:

function A(fn) {
  fn();
}

A(B); // and A(spy) in the test

but in reality A is more complex and I don't really want to add parameters purely for testing. In other JavaScript environments B can be accessed from a global such as window, like in the example from this post:

function bar(x,y) {
  console.debug(x, y);
}
function foo(z) {
  bar(z, z+1);
}
// Spy on the function "bar" of the global object.
var spy = sinon.spy(window, "bar");

Is there an equivalent of this in Deno? I've tried window and self, but those error saying B is undefined on those objects. Also in my project B is first imported from another file, and I've tried import * as foo from "my_file.ts" and then sinon.spy(foo, 'B'), but in that case sinon errors saying ES modules cannot be spied. Can I spy on B without having to rewrite A to take an extra parameter?

[GJZwiers]

Here is what I ended up doing, in case anyone finds it useful. Add the functions A and B to an object and have A call B from the object:

// foo.ts

function A() {
  funcs.B();
}

function B() {

}

const funcs = {
  A,
  B,
};

Export funcs to a .test file and wrap it with sinon to spy on B:

// foo.test.ts

import sinon from "https://cdn.skypack.dev/sinon@12.0.1?dts";
import { funcs } from "./foo.ts";

const spy = sinon.spy(funcs, "B");

// Deno.test something that calls A, then assert that B was called once:
assertEquals(spy.getCalls().length, 1);

[bartlomieju]

@GJZwiers I suggest to upstream this solution to https://github.com/denoland/manual

[GJZwiers]

Sounds good! I will see if I can find some time for it in the coming weeks :)