TypeError('app.use() requires a middleware function')

Issue

enter image description here

here's the app.js(--please find attached image):

//here's the app.js 
const express = require("express");
const app = express();
app.use(express.json());
const morgan = require("morgan");
const mongoose = require("mongoose");
const cors = require("cors");
require("dotenv/config");
const authJwt = require("./helpers/jwt");
const errorHandler = require("./helpers/error-handler");
app.use(cors());
app.options('*', cors());


//middleware
app.use(morgan("tiny"));
app.use(authJwt());
app.use(errorHandler());  //-> **this isn't correct?**


//Importing routing of products
const categoriesRoutes = require("./routes/categories");
const productsRoutes = require("./routes/products");
const usersRoutes = require("./routes/users");
const ordersRoutes = require("./routes/orders");
const req = require("express/lib/request");
const res = require("express/lib/response");



const api = process.env.API_URL;

//routers
app.use(`${api}/products`, productsRoutes);
app.use(`${api}/categories`, categoriesRoutes);
app.use(`${api}/orders`, ordersRoutes);
app.use(`${api}/users`, usersRoutes);

//Database
mongoose
  .connect(process.env.CONNECTION_STRING, {
    useNewurlParser: true,
    useUnifiedTopology: true,
    dbName: "eshop-database",
  })
  .then(() => {
    console.log("database connection is ready");
  })
  .catch((err) => {
    console.log(err);
  });

 
  
//Server
app.listen(3000, () => {
  console.log("Server is Running http://http://localhost:3000");
});

//error image

//here's the error handler code
function errorHandler(err, req, res, next){
    if (err) {
        res.status(500).json({message: err})
    }
}
    module.exports = errorHandler;

> Blockquote (--please find attached image)
here's the error 
C:\Users\steve\Backend\node_modules\express\lib\application.js:210
throw new TypeError('app.use() requires a middleware function')
^

TypeError: app.use() requires a middleware function
at Function.use (C:\Users\steve\Backend\node_modules\express\lib\application.js:210:11)
at Object.<anonymous> (C:\Users\steve\Backend\app.js:17:5)
at Module._compile (node:internal/modules/cjs/loader:1101:14)
at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
at Module.load (node:internal/modules/cjs/loader:981:32)
at Function.Module._load (node:internal/modules/cjs/loader:822:12)    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
at node:internal/main/run_main_module:17:47

Solution

Your error handler function is correct, but the problem is that you're calling the function instead of passing it directly to your express app.

Remove the parenthesis after the errorHandler and it should work.

app.use(errorHandler);  // Don't call errorHandler, express will call it

Think of it like this.

if I just call errorHandler() in any context it won't return anything.

That means that errorHandler() evaluates to undefined.

Now in your code, when you do app.use(errorHandler()) instead of evaluating to the following:

app.use(function(err, req, res, next){
    if (err) {
        res.status(500).json({message: err})
    }
});

Your code is actually evaluating to

app.use(undefined);

and so express is throwing an error because undefined is not a function.

Answered By - Abir Taheer