can you explain which documents outline the parameters that are passed into the express app.METHOD callback function?

Question

can you explain which documents outline the parameters that are passed into the express app.METHOD callback function?

As a beginner in javascript and nodejs, I often struggle with understanding callback functions. One thing that particularly confuses me is determining what arguments are passed into a callback function.

Let's consider the following example:

app.get('/', function (req, res) {
  res.send('GET request to homepage');
});

Is it possible to change the argument names within the function and still have access to the request and response objects? For example:

app.get('/', function (a, b) {
  b.send('GET request to homepage');
});

This works because the app.get method/function defines its second argument as a callback function, which determines what will be passed into that function as arguments.

If this statement holds true, why is it difficult to find documentation in the Express API about what is passed into callback function arguments for different methods/functions? Is there a specific resource where I can find this information, or am I misunderstanding some key terminology?

javascript node.js express callback

Answer 1

Answer №1

If you've stumbled upon this question from 3 years ago in search of an answer (just like I did), then hopefully the response below will brighten your day :)

Typically, callback functions in middleware have three arguments (req, res, next)
- https://expressjs.com/en/guide/writing-middleware.html
Error callbacks follow a different format; they consist of four arguments.
- https://expressjs.com/en/guide/error-handling.html#writing-error-handlers

Writing error handlers
When defining error-handling middleware functions, remember that error-handling functions differ as they require four arguments instead of three: (err, req, res, next). For example:

app.use(function (err, req, res, next) {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

If you're working with TypeScript for your Express API, ensure to explicitly type all parameters to avoid crashing during transpilation due to implicit typing.

app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

Answer 2

If you've stumbled upon this question from 3 years ago in search of an answer (just like I did), then hopefully the response below will brighten your day :)

Typically, callback functions in middleware have three arguments (req, res, next)
- https://expressjs.com/en/guide/writing-middleware.html
Error callbacks follow a different format; they consist of four arguments.
- https://expressjs.com/en/guide/error-handling.html#writing-error-handlers

Writing error handlers
When defining error-handling middleware functions, remember that error-handling functions differ as they require four arguments instead of three: (err, req, res, next). For example:

app.use(function (err, req, res, next) {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

If you're working with TypeScript for your Express API, ensure to explicitly type all parameters to avoid crashing during transpilation due to implicit typing.

app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

Answer 3

Answer №2

The information you seek can be found in the following documentation:

http://expressjs.com/en/api.html#req
http://expressjs.com/en/api.html#res

Answer 4

The information you seek can be found in the following documentation:

http://expressjs.com/en/api.html#req
http://expressjs.com/en/api.html#res

can you explain which documents outline the parameters that are passed into the express app.METHOD callback function?

Answer №1

Answer №2

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