express-middleware-fix.test.js

const express = require("express");
const request = require("supertest");

async function sleep(ms) {
    return new Promise((r) => setTimeout(r, ms));
}

function _500errosync(req, res) {
    throw new Error("Test error");
}

async function _500errorAsync(req, res) {
    await sleep(100);
    throw new Error("Test error async");
}

function interceptErrorMiddleware(err, req, res, next) {
    if (err) {
        console.error("it broke", err);
        res.status(500).send("Something broke!");
    }
}

function _createExpressApp() {
    const app = express();
    fixExpressErrorHandling(app);
    app.get("/test-error-sync", _500errosync);
    app.get("/test-error-async", _500errorAsync);
    app.use(interceptErrorMiddleware);
    return app;
}

function fixExpressErrorHandling(app) {
    app.get = fixRoute(app.get);
    app.post = fixRoute(app.post);
    //etc

    function fixRoute(route) {
        return (path, ...handlers) => {
            const newHandlers = handlers.map((handler) => {
                return async (req, res, next) => {
                    try {
                        return await handler(req, res, next);
                    } catch (err) {
                        next(err);
                    }
                };
            });
            route.call(app, path, ...newHandlers);
        };
    }
}

const app = _createExpressApp();

it("1. Sync error. No problem. Middleware got your back", async () => {
    const response = await request(app).get("/test-error-sync");
    expect(response.status).toBe(500);
    expect(response.text).toBe("Something broke!");
});

it("2. Async error. Middleware doesn't catch it. Unless you fix express", async () => {
    const response = await request(app).get("/test-error-async");
    expect(response.status).toBe(500);
    expect(response.text).toBe("Something broke!");
});