How to convert promise into callback in node js?

Question

Here is simple scenario..

I want to convert these code given in firebase documentation to my api..

How can I convert it to callback function?

var uid = "some-uid";

admin.auth().createCustomToken(uid)
  .then(function(customToken) {
    // Send token back to client
  })
  .catch(function(error) {
      console.log("Error creating custom token:", error);
  });

Here is the link of documentation..

https://firebase.google.com/docs/auth/admin/create-custom-tokens 

Answer 1

If you want to use node-style callbacks on a promise, invoke them like this:

.then(function(result) {
    callback(null, result);
}, function(error) {
    callback(error);
});

Some promise libraries also have helper functions for that, like Bluebirds .asCallback(callback).

Answer 2

In NodeJS > 8.2 you have a require('util').callbackify() (opposite of require('util').promisify()):

const { callbackify } = require('util');

callbackify(() => admin.auth().createCustomToken(uid))((err, customToken) => {
  // ...
});

Or if you have a callback from somewhere else:

const { callbackify } = require('util');

callbackify(() => promise)(callback);

Note that it receives a function that returns a promise and not the promise itself, and it returns a function that receives a callback and does not receive a callback itself.