Express Middlewares execute their arguments, while JavaScript functions doesn't - javascript

I want to write an express middleware, I'm calling it express-stack-player.
What this middleware does is that it gives your middlewares access to the objects (req, res, next), by getting the benefit of the JavaScript scopes ex:
The following is a very simplified version of its code, which sums up its core job of it:
const stackPlayer = (stacks) => (req, res, next) => {
// #param stacks: function
const stacksValue = stacks(req, res, next);
for (middleware of stacksValue) {
middleware(req, res, next);
}
};
It used to work during my project which I'm working on until eventually, I discovered an unwanted behaviour by running the following test:
const express = require("express");
const app = express();
const setter = (req, res, next) => {
console.log("hello world");
req.a = {};
req.a.b = 1;
next();
};
const readSomething = (something) => (req, res, next) => {
console.log(something);
};
const stackPlayer = (stacks) => (req, res, next) => {
// #param stacks: function
const stacksValue = stacks(req, res, next);
for (middleware of stacksValue) {
middleware(req, res, next);
}
};
app.get(
"/",
stackPlayer((req, res, next) => [setter, readSomething(req.a.b)])
);
console.clear();
app.listen(4000, () => console.log("listening on port 4000"));
When I run npm start, it prints the following in the terminal just normally:
But when I request GET /, the following error prints out:
I was testing this JavaScript code in the browser to see its behavior:
const obj = {}
const setOnObj = () => {
obj.a = {}, obj.a.b = 1
}
const readSomething = (arg) => {
console.log(arg)
}
setOnObj()
readSomething(obj.a.b)
If you click on Run code snippet, the output will be 1. So why does my express-stack-player package have a different behavior?
By the way, the package is on npm, and it can be downloaded by running
npm i #samislam/switcher
In the past, I made a bigger package called switcher, but now I want to move the core logic of executing the middlewares into a separate package, and I'll call it express-stack-player (if the name would be available by then).

Related

GET method not working on specific route in ExpressJS

When I try to get data using /:user the function is not running inside the specific routing, can anyone figure me out what is the mistake here?
const express = require("express");
const app = express();
const mongoose = require("mongoose");
const {accountSC,catalogSC} = require("./schema");
let dburl = **database url**
app.use(express.json());
mongoose.connect(dburl);
app.get("/catalog", async (req, res) => {
//some func inside it
});
app.get("/:catalog/:id", async (req, res) => {
//some func inside it
});
app.get("/:user", async (req, res) => {
//some func inside it
});
Basically your /catalog and /:user are the same because of the structure. :user value can be catalog also. So, try out different naming approaches of the routes (/info/:user).
Thank you.
update:
Try out this one.
app.get("/catalog", async (req, res) => {
//some func inside it
});
app.get("/catalog/:catalog/:id", async (req, res) => {
//some func inside it
});
app.get("/info/:user", async (req, res) => {
//some func inside it
});

Chaining of middleware in Express

I am writing APIs and wanted to understand what is a good way to add middleware shared by multiple routes. The middlewares does the same things in each route, like validating hosts/ip, validate user, etc.
The req object gets loaded with other objects in each of the middlewares like req.host, req.ip, req.username etc.
app.post("/route1", middleware1, middleware2, middleware3, middleware4);
app.post("/route2", middleware1, middleware2, middleware3, middleware4);
const middleware1 = (req, res, next) => {
// does something to validate user
req.username = "username"
next();
}
const middleware2 = (req, res, next) => {
// host validation
req.host = "something modified in validation"
next();
}
const middleware3 = (req, res, next) => {
// checks for mac and ip
req.mac = "mac addr"
next();
}
const middleware4 = (req, res, next) => {
res.send();
}
Or something like this:
app.post("/route1", middleware1);
app.post("/route2", middleware1);
const middleware1 = (req, res) => {
// does something to validate user
req.username = "username"
middleware2(req, res);
}
const middleware2 = (req, res) => {
// host validation
req.host = "something modified in validation"
middleware3(req, res);
}
const middleware3 = (req, res) => {
// checks for mac and ip
req.mac = "mac addr"
middleware4(req, res);
}
const middleware1 = (req, res) => {
res.send();
}
Thanks.
Generally I wouldn't call middlewares directly from another middleware. It mixes responsibilities of middleware logic and where the middleware is used.
Express is much more configurable than you think though. You can also install common middlewares in common paths:
If all routes use the middlewares:
// How common middlewares are normally installed:
app.post(middleware1);
app.post(middleware2);
app.post(middleware3);
app.post(middleware4);
// Alternative, less common way to do it:
app.post(middleware1,middleware2,middleware3,middleware4);
If only a specific pattern of urls use the middlewares:
// Use a regexp:
app.post(/route(1|2)/, middleware1, middleware2, middleware3, middleware4);
// Or if you don't like regexp, use globs:
app.post('route*', middleware1, middleware2, middleware3, middleware4);
// Or a more specific glob pattern:
app.post('route1?2?', middleware1, middleware2, middleware3, middleware4);
If all url in a subpath use the middlewares. For example, lets say if all urls in /route/... use the middlewares:
const route = express.Router();
app.use('/route',route);
route.post(middleware1);
route.post(middleware2);
route.post(middleware3);
route.post(middleware4);
If none of the above appeal to you you can still use your second option but instead of calling middlewares inside each other you write a middleware to initialize middlewares:
function commonMiddlewares (req, res, next) {
middleware1(req,res,function() {
middleware2(req,res,function() {
middleware3(req,res,function() {
middleware4(req,res,next);
});
});
});
}
Which can be written in a less nested way:
function commonMiddlewares (req, res, next) {
function runMiddleware4 () {
middleware4(req,res,next);
}
function runMiddleware3 () {
middleware3(req,res,runMiddleware4);
}
function runMiddleware2 () {
middleware2(req,res,runMiddleware3);
}
middleware1(req,res,runMiddleware2);
}
const express = require('express')
const { routesMiddleware } =require('./middlewares')
const { pureVaidationsFunctions1 } =require('./services')
const rout1 =express.Router()
const rout2 =express.Router()
const app = express()
app.use('/route1',route1)
app.use('/route2',route2)
// routesMiddleware a middleware to handle the execution of list of functions
// pureVaidationsFunctions1 list of funtions that `routesMiddleware` will consume
route1.post(routesMiddleware(pureVaidationsFunctions1))
route2.post(routesMiddleware(pureVaidationsFunctions2))
make sense?
You can specify multiple middlewares, see the app.use docs:
An array of combinations of any of the above.
You can create a file of all middlewares like -
middlewares.js
module.exports = [
function(req, res, next){...},
function(req, res, next){...},
function(req, res, next){...},
.
.
.
function(req, res, next){...},
]
and as then simply add it like:
/*
you can pass any of the below inside app.use()
A middleware function.
A series of middleware functions (separated by commas).
An array of middleware functions.
A combination of all of the above.
*/
app.use(require('./middlewares.js'));
Note - Do this only for those middlewares which will be common for all such requests.

How to get data in one middleware from another?

I have two middleware functions attached to my app get request which works fine.
const express = require('express');
const app = express();
function fun1 (req, res, next) {
console.log('this is fun1')
next()
}
function fun2 (req, res, next) {
console.log('this is fun2')
next()
}
app.get('/', fun1, fun2, function (req, res, next) {
res.send('User Info')
})
app.listen(8080, () => console.log(`Listening on port 8080!`))
Now if I try to do next('test') in fun1 then it bypass fun2 and does 'test' output in browser window instead of 'User Info' which is correct. But how do I get data in fun2? I need to pass something from fun1 and get it in fun2 for further validation.
Assign it to req. You will have access to the same request and response objects through all middlewares.
Note that next('test') does not respond to the client or at least it is not meant to. It is meant to handle errors. Without an error handler and in development mode, Express shows these errors in the browser.
Read on:
Error handling in Express
You can do this by attaching a key-value pair with req` object.
Now how to do this,
const express = require('express');
const app = express();
function fun1 (req, res, next) {
req.MY_VAR = 'MY_VAL'; // setting the value
console.log('this is fun1')
next()
}
function fun2 (req, res, next) {
let myVar = req.MY_VAR; // retrieving the value
console.log(myVar); // MY_VAL
console.log('this is fun2')
next()
}
app.get('/', fun1, fun2, function (req, res, next) {
res.send('User Info')
})
app.listen(8080, () => console.log(`Listening on port 8080!`))
Now, why not next()? Generally, the value passed in next() will be received by the error argument in app.get('/', function (err, req, res, next) {} );

Node JS async/await controller issue

I am trying to use an asynchronous function in my Node API controller, but am receiving an error from my 'error-handler' middleware.
TypeError: fn is not a function
at eval (webpack:///./app/middleware/errorHandler.js?:16:21)
It does not like my 'findAll' function exported from my controller, why is this not a function? Am I exporting the function correctly? Am I using async/await correctly? Do I need a polyfill for this? I understood that async/await was supported from Node v8. I am currently running Node v11.10 and Express v4.16.4.
Here is my routes file:
// routes.js
const verifyToken = require('../../middleware/verifyToken.js');
const errorHandler = require('../../middleware/errorHandler.js');
module.exports = app => {
const controller = require('../../controllers/controller.js');
app.get(`/collection`, verifyToken, errorHandler(controller.findAll));
}
Here is my controller:
// controller.js
exports.findAll = async (req, res) => {
const collections = await collection.find().populate('other');
res.send(collections);
};
Here is my middleware:
// errorHandler.js
module.exports = fn => {
return (req, res, next) => {
Promise.resolve(fn(req, res, next)).catch(next);
};
};
Any help is greatly appreciated.
Im not sure but is errorHandler expecting fn to be the error? If so, why is it called passing (req, res next)?
I use following structure:
Router
// routes.js
const verifyToken = require('../../middleware/verifyToken.js');
const controller = require('../../controllers/controller.js');
var router = express.Router()
router.route('/collection').get(
verifyToken,
controller.findAll
)
module.exports = router
Controller
// controller.js
const asyncUtil = fn =>
function asyncUtilWrap(req, res, next, ...args) {
const fnReturn = fn(req, res, next, ...args)
return Promise.resolve(fnReturn).catch(next)
}
module.exports = {
findAll: asyncUtil(async (req, res, next) => {
const collections = await collection.find().populate('other'); // you can do try/catch here if you want to handle the error here
res.send(collections);
};
Then Error Handler usually goes at bottom of app.js (but you can place it at bottom of your router):
// app.js
app.use(function(err, req, res, next) {
res.status(err.status || 500)
res.send(err.message)
})
I believe this is how I would do it if I understand you correctly:
// routes.js
const verifyToken = require('../../middleware/verifyToken.js');
const controller = require('../../controllers/controller.js');
module.exports = app => {
app.get(`/collection`, verifyToken, controller.findAll);
}
// controller.js
exports.findAll = async (req, res, next) => {
try {
const collections = await collection.find().populate('other');
res.send(collections);
} catch(err) {
console.log(err); // up to you what to do with the error
next();
}
};

Express.js - wrap every middleware/route in "decorator"

I have Express.js instance and couple of routes which I want to wrap in some function. Example:
const wrapper = (route) => {
return (req, res, next) => {
let result = route(req, res, next);
// do some independent processing
}
};
app.get('/', wrapper((req, res, next) => {
// respond to request somehow
}));
While this works fine, I don't like the idea to explicitly call wrapper on every route or middleware which requires such processing.
Is there any way to be able to wrap every required route/middleware in certain wrapper (given that wrapper function can check that this route/middleware needs to be wrapped) implicitly (via Express.js extension, monkey-patching or some special middleware)?
UPDATE:
More solid example. Let's assume I want to make an async router functions. But I don't want to catch errors in each and every route function. So I wrap them up:
const wrapper = func => (req, res, next) => {
const promise = func(req, res, next);
if (promise.catch) {
promise.catch(err => next(err));
}
next();
};
app.get('/one', wrapper(async (req, res, next) => {
// respond to request somehow
}));
app.get('/two', wrapper(async (req, res, next) => {
// respond to request somehow
}));
app.get('/three', wrapper(async (req, res, next) => {
// respond to request somehow
}));
// and so on...
app.use((err, req, res, next) => {
// do something with intercepted error
});
This explicit wrapper around all routes is actually the thing I want to get rid of.
It turned out to be a bit of a PITA because, ultimately, Express doesn't propagate the return value of a route handler function.
This is what I came up with (a monkey-patch):
const Layer = require('express/lib/router/layer');
const handle_request = Layer.prototype.handle_request;
Layer.prototype.handle_request = function(req, res, next) {
if (! this.isWrapped && this.method) {
let handle = this.handle;
this.handle = function(req, res, next) { // this is basically your wrapper
let result = handle.apply(this, arguments);
// do some independent processing
return result;
};
this.isWrapped = true;
}
return handle_request.apply(this, arguments);
};
I would probably suggest using a similar approach as express-promise-router though, which implements a drop-in replacement for Express' Router. However, it's not implicit.
Why not just use next()?
You can add stuff on req like
app.get('/', (req, res, next) => {
req.somestupidfieldthatidontevenknowwhyinamedthisway = 42;
next();
});
app.get('/', (req, res, next) => {
//req.somestupidfieldthatidontevenknowwhyinamedthisway is now accessible as 42
var valueFromPreviousMiddleware = req.somestupidfieldthatidontevenknowwhyinamedthisway;
.....
});
You could wrap middleware and router as below
function wrapper(func) {
return function inner(req, res, next) {
const start = Date.now();
func(req, res, function () {
let elapsedMS = Date.now() - start
console.log('time elapsed for function ' + func.prototype.constructor.name + ' is ' + elapsedMS)
next.apply(this, arguments);
});
};
}
var originalAppUse = app.use;
app.use = function () {
lastArg = arguments.length - 1;
if (typeof arguments[lastArg] === 'function') {
arguments[lastArg] = wrapper(arguments[lastArg])
}
originalAppUse.apply(this, arguments)
}

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