I am learning express and mongo and I came across a line mongoose.Promise = global.Promise which I don't understand where the global.Promise came from?

Following is the code

import express from "express";
import routes from "./src/routes/crmRoutes";
import mongoose from "mongoose"
let app = express();
let PORT = 9666;

routes(app)

app.get("/", (req, res)=>{
    res.send("hello from node and express")
    console.log(global.Promise)
})