Basic Syntax <\/strong><\/p>\n An Express application can use any of the below mentioned types of middleware:<\/p>\n Application-level middleware : app.use<\/p>\n<\/li>\n Router-level middleware : router.use<\/p>\n<\/li>\n Error handling middleware app.use(err,req,res,next)<\/p>\n<\/li>\n Built-in middleware : express.static,express.json,express.urlencoded<\/p>\n<\/li>\n Third-party middleware : bodyparser,cookieparser<\/p>\n<\/li>\n<\/ul>\n Error-handling middleware will have four arguments, the forth next will identify it as an error-handling middleware. the signature is (err, req, res, next). Express error handling middle-ware is very helpful in catching globally occurring errors without worrying about the Server to crash.<\/p>\n To generate the run-time error, we will throw random error and see whether it catches it or not.<\/p>\n In some cases we will be adding some extra features to our backend<\/p>\n Install the Node.js module for the required functionality, then load it in your app at the application level or at the router level<\/p>\n Example : body-parser <\/p>\n This third party middleware will Parse incoming request bodies in a middleware before your handlers, available under the req.body property.<\/p>\n To install run the below command <\/p>\n Example : app.js<\/p>\n Run the command in node terminal<\/p>\n And browse the path in postman : “http:\/\/localhost:3000\/save” <\/p>\n Output<\/p>\n when a route does not match with any handler then it will throw 404 Error. To do this we will put a middle-ware at the end of all routes, it will be executed when none of the routes match.<\/p>\n We’re distinguishing between errors in development and in production. When we’re in production, we send the error using this function here, which will then send as many details as possible to the client, so that we really get all the information in order to get rid of the bug. If it’s a programming error, or if it’s an operational error, then we still really want to see anything that’s going on. If the error is operational, so for example On the other hand, we have these unknown errors\/unexpected errors 1. Invalid ID (We have to show it as Operational Errors) – (error.name === ‘CastError’) 500 – Internal Server Error<\/p>\n Steps: # move the middleware from index.js to appController.js<\/strong><\/p>\n # async function return promises so receive the error in catch part.<\/p>\n Error-handling middleware Basic Syntax let convertRupees = (dollar) => { if(typeof dollar === ‘number’) { return dollar * 69 } else { throw Error (‘Amount must be in number’) } } try { console.log(convertRupees(‘one’)); } catch (error){ console.error(error) } console.log(‘If we used try…catch..This line will show even if error raised<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[12,1],"tags":[37,35,34],"class_list":["post-209","post","type-post","status-publish","format-standard","hentry","category-mongodb","category-nodejs","tag-error-handling-middleware","tag-express-js-middleware","tag-express-middleware","ct-col-2"],"_links":{"self":[{"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/posts\/209","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/comments?post=209"}],"version-history":[{"count":6,"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/posts\/209\/revisions"}],"predecessor-version":[{"id":2240,"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/posts\/209\/revisions\/2240"}],"wp:attachment":[{"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/media?parent=209"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/categories?post=209"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uitutorials.in\/wp\/wp-json\/wp\/v2\/tags?post=209"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}let convertRupees = (dollar) => {\r\n\tif(typeof dollar === 'number') {\r\n\t\treturn dollar * 69\r\n\t} else {\r\n\t\tthrow Error ('Amount must be in number')\r\n\t}\r\n}\r\n\r\ntry {\r\n\tconsole.log(convertRupees('one'));\r\n} catch (error){\r\n\tconsole.error(error)\r\n}\r\n\r\nconsole.log('If we used try...catch..This line will show even if error raised ')\r\n<\/code><\/pre>\n\n
Example Without Error-handling Middleware<\/h3>\n
\r\nvar express = require('express');\r\nvar app = express();\r\n\r\napp.use(function(req,res,next) {\r\n console.log(\"In second route\");\r\n next(); \/\/ go to next route.\r\n});\r\n\r\napp.get('\/', function(req,res){\r\n res.send('Welcome NodeJs !');\r\n})\r\nserver = app.listen(3000, function() {\r\n console.log('Listenint Port : 3000 ');\r\n });\r\n<\/code><\/pre>\nExample Error-handling Middleware<\/h3>\n
\r\nvar express = require('express');\r\nvar app = express();\r\n\r\napp.use(function(req,res,next) {\r\n console.log(\"In second route\");\r\n next(); \/\/ go to next route.\r\n});\r\n\r\napp.get('\/', function(req,res){\r\n throw new Error(); \/\/ Express will catch this on its own.\r\n res.send('Welcome NodeJs !');\r\n})\r\napp.use(function(err,req,res,next) {\r\n console.log(\"Error happens\",err.stack);\r\n res.status(500).send({\"Error Happen in the Request : \" : err.stack});\r\n});\r\nserver = app.listen(3000, function() {\r\n console.log('Listenint Port : 3000 ');\r\n });\r\n <\/code><\/pre>\nThird-party Middlewares<\/h3>\n
npm install body-parser <\/code><\/pre>\n \r\nvar express = require('express');\r\nvar bodyParser = require('body-parser');\r\nvar app = express();\r\n\r\napp.use(bodyParser.urlencoded({extended:false}))\r\n\r\napp.use(bodyParser.json())\r\n\r\napp.post('\/save',(req,res)=>{\r\n res.json({\r\n \"status\":true,\r\n \"payload\":req.body\r\n })\r\n})\r\n \r\nvar server = app.listen(3000, function() {\r\n console.log('Listenint Port : 3000 ');\r\n });\r\n<\/code><\/pre>\nnode app.js<\/code><\/pre>\n\r\n{\r\n \"status\": true,\r\n \"payload\": {}\r\n}\r\n<\/code><\/pre>\nHandling 404 errors with Router middleware <\/h3>\n
\r\n\r\nvar express = require(\"express\");\r\nvar app = express();\r\n\r\n\/\/Creating Router() object\r\n\r\nvar router = express.Router();\r\n\r\n\/\/ Router middleware, mentioned it before defining routes.\r\n\r\nrouter.use(function (req, res, next) {\r\n console.log(\"\/\" + req.method);\r\n next();\r\n});\r\n\r\nrouter.use(\"\/user\/:id\", function (req, res, next) {\r\n console.log('Passed Value : ' + req.params.id)\r\n if (req.params.id == 0) {\r\n res.json({ \"message\": \"Pass ID other than 0\" });\r\n }\r\n else next();\r\n});\r\n\r\n\/\/ Provide all routes here, this is for Home page.\r\n\r\nrouter.get(\"\/\", function (req, res) {\r\n res.json({ \"message\": \"Welcome Express Router\" });\r\n});\r\n\r\nrouter.get(\"\/user\/:id\", function (req, res) {\r\n res.json({ \"message\": \"Express Router with Param \" + req.params.id });\r\n});\r\n\r\n\/\/ Handle 404 error. \r\n\/\/ The last middleware.\r\napp.use(\"*\", function (req, res) {\r\n res.status(404).send('404');\r\n});\r\n\r\n\r\n\/\/ Tell express to use this router with \/api before ex - http:\/\/localhost:3000\/api .\r\napp.use(\"\/api\", router);\r\n\r\n\/\/ Listen to this Port\r\n\r\napp.listen(3000, function () {\r\n console.log(\"Listening Port : 3000\");\r\n});\r\n<\/p>\r\n<\/code><\/pre>\nError Handling<\/h2>\n
\nWhen we’re in production, which is arguably the most important part of our application, then we distinguish between operational errors, so errors that we know and trust, and to other errors, that might be kind of unexpected. <\/p>\n
\n1.the user tried to access a route that doesn’t exist,
\n2.or tried to input invalid data
\nall of these are operational errors.Then we can send appropriate error messages, for the client to know what went wrong.<\/p>\n
\nFor example : Coming from MongoDB, which we do not mark as operational. In this case, they would right now simply be handled using this generic error message here. For example, a validation error. Right now, that’s an error that’s coming from MongoDB and not from our own app error class. We do not create these errors by ourselves. Again, they are right now not marked as operational, but we of course need to mark them as operational so that we can then send the appropriate error message back to the client. In the example that I was just mentioning, that the input data is invalid. There are two or three other errors that we need to mark as operational ourselves. So we will do that down here. So in this else block, we will do that over the next couple of lectures.<\/p>\nThree type of Errors in MongoDB : 400 – Bad Request<\/h3>\n
\n2. Duplicate Name In DB (error.code == 1100)
\n3. ValidationError [Max Value] – (error.name == ‘ValidationError’)<\/p>\n
\n=======<\/p>\nWe can directly create the middleware in “index.js”<\/h3>\n
app.all('*', (req, res, next) => {\r\n \r\n res.status(404).json({\r\n status: 'fail',\r\n message: `Can't Find URL${req.originalUrl} on the server`\r\n })\r\n \r\n \/\/ const err = new Error(`Can't find ${req.originalUrl} on this server !`)\r\n \/\/ err.status = 'fail';\r\n \/\/ err.statusCode = 404;\r\n \/\/ next(err);\r\n\r\n \r\n})\r\n\r\napp.use((err, req, res, next)=> {\r\n err.statusCode = err.statusCode || 500;\r\n err.status = err.status || 'error';\r\n\r\n res.status(err.statusCode).json({\r\n status: err.status,\r\n message: err.message\r\n });\r\n})\r\n<\/code><\/pre>\n2. we’re gonna build a couple of different functions for handling with different types of errors,and so I want all of these functions to be all in the same file, all right ? Controlling errors (appController.js)<\/h3>\n
module.exports = (err, req, res, next)=> {\r\n err.statusCode = err.statusCode || 500;\r\n err.status = err.status || 'error';\r\n\r\n res.status(err.statusCode).json({\r\n status: err.status,\r\n message: err.message\r\n });\r\n}<\/code><\/pre>\n4. import “appController” into the “index.js” file<\/h3>\n
const globalErrorHandler = '.\/appController.js'\r\n\r\napp.all('*', (req, res, next) => {\r\n next(new AppError(`Can't Find URL${req.originalUrl} on the server`, 404));\r\n})\r\napp.use(globalErrorHandler)<\/code><\/pre>\n3. Add the AppError Class<\/h3>\n
class AppError extends Error {\r\n constructor(message, statusCode){\r\n super(message);\r\n this.statusCode = statusCode;\r\n this.status = `${statusCode}`.startsWith('4') ? 'fail':'error';\r\n this.isOperational = true;\r\n\r\n Error.captureStackTrace(this, this.constructor);\r\n }\r\n}\r\n\r\nmodule.exports = AppError;<\/code><\/pre>\n4. import “AppError” into the “index.js” file<\/h3>\n
app.all('*', (req, res, next) => {\r\n next(new AppError(`Can't Find URL${req.originalUrl} on the server`, 404));\r\n})<\/code><\/pre>\n5.Modify the async await functions<\/h3>\n
const catchAsync = fn => {\r\n\treturn (req,res,next) => {\r\n\t\tfn(req, res, next).catch(err=> next(err))\r\n\t}\t\r\n}\r\n\r\nexport.createTour = catchAsync(async(req,res,next) => {\r\n\tconst newTour = await Tour.create(req.body)\r\n\t\r\n\tres.statu(201).json({\r\n\t\tstatus: 'Success',\r\n\t\tdata: {tour: newTour}\r\n\t})\r\n})<\/code><\/pre>\n6. Move the catchAsync to new file <\/h3>\n
module.exports = catchAsync = fn => {\r\n\treturn (req,res,next) => {\r\n\t\tfn(req, res, next).catch(err=> next(err))\r\n\t}\t\r\n}<\/code><\/pre>\n7. Import catchAsync into controller.js and use it<\/h3>\n
<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"