Mongoose - Why we make "mongoose.Promise = global.Promise" when setting a mongoose module?

This is legacy code from older examples that isn't needed with Mongoose 5.

Mongoose 4 relied on its own promise implementation, mpromise. mongoose.Promise wasn't necessarily Promise global.

As Mongoose 4 documentation states:

Mongoose 5.0 will use native promises by default (or bluebird, if native promises are not present) but still support plugging in your own ES6-compatible promises library. Mongoose 5.0 will not support mpromise.

Though the statement about Bluebird is no longer true; Mongoose 5 dropped the support of Node versions that don't have native promises.

mongoose.Promise = global.Promise;

may still be needed if global.Promise was assigned with another implementation (e.g. Bluebird) after Mongoose was imported, though a better thing would be to assign global.Promise = Bluebird earlier instead.

Mongoose maintainer here. If you're using Mongoose 5, please remove mongoose.Promise = global.Promise;. That line was used to address the below deprecation warning with promises in Mongoose 4:

WARNING: Mongoose: mpromise (mongoose's default promise library) is deprecated, plug in your own promise library instead

It does nothing in Mongoose 5. You should only use mongoose.Promise in Mongoose 5 if you want to use your own promise library with Mongoose, like Bluebird or Q.

There's also a little more about mongoose.Promise here:

If we want to use mongoose in different position inside the codes it must be viewed as global mode, that's why we need to set mongoose as :

mongoose.Promise = global.Promise;