greet is not a function (However It declared in mongoose file as a Schema method) (JS) -

Question

this is the mongoose file that I run through Node:

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/shopApp')
.then(()=>{
    console.log('CONNECTION OPEN!!!')
})
.catch(err=>{
    console.log('HO NO ERROR!!!')
    console.log(err)
})

// alternative syntax for Schema, now we can add additional information. the require.
const productSchema = new mongoose.Schema({
    name: {
        type: String,
        required: true,
        maxlength: 20   // Schema options
    },
    price: {
        type: Number, // number character inside a string will be accepted and the 
opposite also!!!!!!!!!!
        min: [0, 'Price must be positive'] // minimum character from SchemaTypes options 
and then error msg
    },
    onSale: {
        type: Boolean,
        default: false
    },
    categories: {
        //[String] which means: it should be an array only consisting of strings
    type: [String]

    }, // 
    qty: {
        online: {
            type: Number,
            default: 0
        },
        inStore: {
            type: Number,
            default: 0
        }
    },
    size: {
        type: String,
        enum: ['S', 'M', 'L'] // can be only one of this values, else it return an error
    }
})


const product = mongoose.model('product', productSchema);
//instance methods
// someSchema.methods.findSimilarType = function() {
    //...
//}

productSchema.methods.greet = function(){
    console.log('HELLO!!! HI!!! HOWDY!!!')
} // we want to use regular function

/*const bike = new product({name: 'Cycling Jersy', price: 18.50, categories: ['Cycling', 
' Safety'], size: 'XL'}) // the mongo will ignore the color property
bike.save()
.then(data=>{
    console.log('IT WORKED!')
    console.log(data)
})
.catch(err=>{
    console.log('HO NO ERROR!!!')
    console.log(err)
})*/

product.findOneAndUpdate({name: 'Tire pump'}, {price: 1}, {new: true, runValidators: 
true})  // Schema constraints don't work with updating ( it below minimum price),
// you need to set ---> runValidators: true!!!!!
.then(data=>{
    console.log('IT WORKED!')
    console.log(data)
})
.catch(err=>{
    console.log('HO NO ERROR!!!')
    console.log(err)
})

---

in node.js I loaded this mongoose file and created an instance of the model and tried to run the function greet on it.

const p = new product({name: 'Big Bag', price: 10})
p.greet()

In the background mongod and mongo are running both on the prompt.

I get a message 'greet is not a function'.