What does `app.use(bodyParser.json())` do?

Question

For:

bodyParser.urlencoded({extended: ...})

my research shows me that if extended: true, then you can parse nested objects, or generally any type. However, if you set extended: false, then you can only parse strings or arrays. But what does ...

app.use(bodyParser.json())

mean exactly? I mean, yes... I know the docs mention that it parses json. But I am still confused. I have noticed applications that set extended: true do NOT use bodyParser.json() at all. But applications that extended: false tend to use bodyParser.json(). Why is this? At the end of the day, both applications are able to parse json.

Secondly, which is the recommended approach?

Answer 1

Okay, contrary to what I previously thought, further research shows that extended: true and app.use(bodyParser.json()) can be used together. So it is not only extended: false that uses it. The statement app.use(bodyParser.json()) is to be used independently, whether you set extended as true or false.

• app.use(bodyParser.json()) basically tells the system that you want json to be used.

• bodyParser.urlencoded({extended: ...}) basically tells the system whether you want to use a simple algorithm for shallow parsing (i.e. false) or complex algorithm for deep parsing that can deal with nested objects (i.e. true).

Have a look at the docs (i.e. https://expressjs.com/en/guide/migrating-4.html) for the example.

Answer 2

URL encoding and JSON encoding both allow to convert a (nested) object to string, but the format is different. An URL encoded string is in general not a valid JSON string.

One application may use one encoding method, and another the other. As long as they don't mix the two, it will work.

Answer 3

bodyParser.json returns middleware that only parses json. This parser accepts any Unicode encoding of the body and supports automatic inflation of gzip and deflate encodings.

A new body object containing the parsed data is populated on the request object after the middleware (i.e. req.body).