Prevent common pitfalls and promote best practices when using ExpressJS

You'll first need to install ESLint:

$ npm i eslint --save-dev

Next, install eslint-plugin-express:

$ npm install eslint-plugin-express --save-dev

Note: If you installed ESLint globally (using the -g flag) then you must also install eslint-plugin-express globally.

Add express to the plugins section of your .eslintrc configuration file. You can omit the eslint-plugin- prefix:

{
    "plugins": [
        "express"
    ]
}

Then configure the rules you want to use under the rules section.

{
    "rules": {
        "express/error-middleware-4-params": 2
    }
}

Express looks at the number of params the middleware takes to determine whether it is normal middleware (3) or error middleware(4 (Express documentation). A common mistake is to remove the next param when defining error middleware.

Examples of incorrect code for this rule:

/*eslint express/error-middleware-4-params: "error"*/

app.use(function (error, req, res) {})
app.use(function (err, req, response) {})
app.use(function (e, request, resp) {})

// etc.

Examples of correct code for this rule:

/*eslint express/error-middleware-4-params: "error"*/

app.use(function (req, res, next) {});
app.use(function (error, req, res, next) {});
app.use(function (e, request, resp, next) {});

// etc.

NB: this rule looks at the function param names, the following names are recognized:

• 1st param: error, err, e
• 2nd param: request, req
• 3rd param: response, resp, res