Why the error is occurred??

const Schema = mongoose.Schema;
const AnimalSchema = new Schema({
    type: {type: String, default: "goldFish"},
    color: {type: String, default: "golden"},
    size: String,
    mass: {type: Number, default: 0.006},
    name: {type: String, default: "Simon"},
});

AnimalSchema.pre('save', next => {
    if(this.mass >= 100) {
        this.size = 'big';
    } else if(this.mass >= 5 && this.mass < 100) {
        this.size = 'mdeium';
    } else {
        this.size = 'small';
    }
    next();
});

let Animal = mongoose.model('Animal', AnimalSchema);
let elephant = new Animal({
    type: "elephant",
    color: "gray",
    mass: 6000,
    name: "David"
});
let animal  = new Animal({}); // glodfish
let whale = new Animal({
    type: "whale",
    mass: 190500,
    name: "Angela"
});

const animalData = [
    {
        type: "mouse",
        color: "gray",
        mass: 0.035,
        name: "Micky"
    },
    {
        type: "nutria",
        color: "brown",
        mass: 0.12,
        name: "Johny"
    },
    {
        type: "wolf",
        color: "yellow",
        mass: 35,
        name: "Ghill"
    },
    elephant,
    animal,
    whale
];
Animal.remove({}, (err) => {
    if(err) console.error(err);
    Animal.create(animalData, (err, animals) => {
        if(err) console.error(err);
        Animal.find({}, (err, animals) => {
            animals.forEach(animal => {
                console.log(animal.name + " the " + animal.color +
                    " " + animal.type + " is a " + animal.size + "-sized animal.");
            }); // animals.forEach
        }); // Animal.find
        db.close(() => console.log('db connection closed'));
    }); //Animal.create
}); //Animal.remove