Fake Imports

This is a small module for Deno and browser environments that can be used for blocking specific imports using the esm import syntax. It is mostly meant for Stubbing and Mocking modules in unit tests, but can be used for other purposes as well.

Take the following two files for example:

// module_with_side_effects.js
const div = document.createElement("div");

export function getDiv() {
  return div;
}

// foo.js
import { getDiv } from "./module_with_side_effects.js";

export class Foo {
  x = 1;
  div = null;

  getX() {
    return this.x;
  }

  setupElement() {
    this.div = createDiv();
  }
}

Let’s say we want to write a unit test for the getX() method:

// foo.test.js
import { Foo } from "./foo.js";
import { assertEquals } from "https://deno.land/std@0.118.0/testing/asserts.ts";

Deno.test("getX", () => {
  const foo = new Foo();
  const x = foo.getX();
  assertEquals(x, 1);
});

In theory this should work, because in our test the div isn’t used. But if we run deno test we get:

error: Uncaught ReferenceError: document is not defined
const div = document.createElement("div");
            ^

Because module_with_side_effects.js has side effects, the div is created the moment the module is imported. Ideally a problem like this would prompt you to rewrite your code so that no modules have any side effects, but this isn’t always feasible.

This module was created to solve this problem. By allowing you to overwrite the code of an imported file, you can change its behaviour, or prevent it from doing anything at all.