Replacing the Undici global dispatch logic with a symbol-less Jest-compatible implementation

my-test.spec.js

test("mocking a request", async () => {
  const mockAgent = new MockAgent()
  ;(global as any).setMockedFetchGlobalDispatcher(mockAgent)
  
  await fetch(...)
})

undici-mockagent-fetch-jest.js

const { TestEnvironment } = require("jest-environment-node")
const { fetch: undiciFetch, MockAgent } = require("undici")
const v8 = require('node:v8');

module.exports = class extends TestEnvironment {
    constructor(config, context) {
        super(config, context)
        this.mockGlobalFetch()
    }

    /**
     * Replace the global fetch implementation with one which still uses undici, but pulls
     * in the correct "dispatcher" from the global object.
     *
     * Background:
     * 1. Every call to fetch uses a "dispatcher", i.e. the underlying HTTP client
     * 2. If you don't explicitly specify it when calling `fetch(...)`, Undici / Node
     *    internally uses a special symbol to find the default global one inside the
     *    global namespace [1]
     * 3. Jest runs each test in a separate process, so Symbol.for(...) returns a
     *    different value in the test process.
     * 4. This means that calling `setGlobalDispatcher(...)` in the test has no effect on
     *    the global version that fetch otherwise would use.
     * 5. This means that your mocks won't apply and won't be cleared up automatically.
     *
     * This method overrides the global fetch, to specify our own global dispatcher, and
     * creates a special method that can be used to replace the dispatcher.
     *
     * [1]: https://github.com/nodejs/undici/blob/e5c9d703e63cd5ad691b8ce26e3f9a81c598f2e3/lib/global.js#L5
     */
    mockGlobalFetch() {
        this.global.fetch = (input, init) => undiciFetch(input, {
            dispatcher: this.global["mockAgent"],
            ...init,
        })

        this.global.setMockedFetchGlobalDispatcher = (agent) =>
            Object.defineProperty(this.global, "mockAgent", {
                value: agent,
                writable: true,
                enumerable: false,
                configurable: false
            })
    }
}

@nickcampbell18 This saved my day, thank you!