Why can't express send using this promise syntax?

Question

Normally when using Promise syntax, the below syntaxes produce the same result:

// Syntax A - works fine
getUser(id).then((user) => console.log(user)

// Syntax B - works fine
getUser(id).then(console.log)

Yet when we attempt this in an Express route, only syntax A works:

// Works
app.get('/syntax_a', (req, res) => {
  getUser(req.body.id).then((user) => res.json(user))
})

// Fails
app.get('/syntax_b', (req, res) => {
  getUser(req.body.id).then(res.json)
})

Syntax B yields an error:

UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'app' of undefined
    at json (/server/node_modules/express/lib/response.js:256:18)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)

Why isn't this working like the first example?

[How to access the correct `this` inside a callback](https://stackoverflow.com/questions/20279484/how-to-access-the-correct-this-inside-a-callback) — Andreas, Sep 04 '21 at 14:20
When you pass a method of an object (like `res.json`), you break the relationship between the method (the `json()` method) and the object (`res`). That means that inside that method, `this` won't work properly. — Pointy, Sep 04 '21 at 14:20
In addition to the comments above, following change will fix the issue: `res.json` ---> `res.json.bind(res)` — Yousaf, Sep 04 '21 at 14:21
And why does it work with console? Because internally `console.log` has no dependency on `this`. — deceze, Sep 04 '21 at 14:26

Sean Vieira · Accepted Answer · 2021-09-04T14:27:04.160

The reason is because response.json is a function that looks up app on this. When you pass res.json you're passing json without any reference to the object it comes from so its this is (most likely) undefined). this in JavaScript is dynamically bound, rather than statically bound as it is in a language like C# or Java.

console.log on the other hand doesn't use this and so isn't affected by how you call it.

We can build something like this ourselves:

let _privateImplementation = (...args) => console.log(...args);

let ourPrivateBoundExample = {
  doIt(some, info) {
    _privateImplementation("Doing it", some, info);
  }
}

ourPrivateBoundExample.doIt(1, 2); // Doing it 1 2
Promise.resolve("Hello there!")
  .then(ourPrivateBoundExample.doIt) // Doing it Hello there!

Why can't express send using this promise syntax?

1 Answers1