I have a product code that injects javascript into document head. I want to write unit test for it. The problem is that jest fails with timeout error. I understand it’s the resolve method of Promise that needs to be called, but I am unable to figure out how will I do it with jest. I tried to create document from JSDOM, but in that approach as well, the promise will not be resolved.

snippet from product code

export async function injectScript() {
    await new Promise((resolve, reject) => {
        const element = document.createElement("script");
        element.setAttribute("type", "text/javascript");
        element.setAttribute("src", `/mycode.js`);

        document.getElementsByTagName("head")[0]?.appendChild(element); // head should not be null
        element.onload = resolve;
        element.onerror = reject;
    });
    return "test";
}

my test code

import { injectScript } from "../MyUtil";
describe("MyFunction", () => {
    it("should return the expected result", async () => {
        const result = await injectScript();
        expect(result).toBe("test");
    });
});

I get error thrown: “Exceeded timeout of 5000 ms for a test. But this will not be solved by increasing timeout.

How will I mock or use JSDOM or other mechanism so that onload is called after document is injected and the promise resolves in my test code?