Mongoose Sandbox: TypeError: Cannot read property 'length' of undefined

Question

When i run node mongoose_sandbox.js, i get the following error:

...\node_modules\kareem\index.js:51 } else if (pre.fn.length > 0)

TypeError: Cannot read property 'length' of undefined

'use strict';

// mongoose setup
const mongoose = require('mongoose');
mongoose.connect("mongodb://localhost:27017/sandbox");
const db = mongoose.connection;

// error handling
db.on("error", (err) => {
    console.error("connection error:", err)
});

db.once("open", () => {
    console.log("db bağlandı, haberin olsun");
    // database work
    const Schema = mongoose.Schema; 
    const AnimalSchema = new Schema({
        type: {type: String, default: "goldfish"},
        size: String,
        color: {type: String, default: "golden"},
        mass: {type: Number, default: "0.007"},
        name: {type: String, default: "Angela"}
    });

AnimalSchema.pre("save"), function (next) {
    if (this.mass >= 100) {
        this.size = "büyük";
    } else if (this.mass >=5 && this.mass < 100) {
        this.size = "orta";
    } else {
        this.size = "küçük";
    }
    next();
};

const Animal = mongoose.model("Animal", AnimalSchema);

const elephant = new Animal({
    type: "fil",
    color: "gri",
    mass: 6000,
    name: "Kutay"
});

const animal = new Animal({});
const whale = new Animal({
    type: "whale",
    mass: 190500,
    name: "Enes"
});

const animalData = [
    {
        type: "mouse",
        color: "gray",
        mass: 0.035,
        name: "Marvin"
    },
    {
        type: "nutria",
        color: "brown",
        mass: 6.35,
        name: "Gretchen" 
    },
    {
        type: "wolf",
        color: "gray",
        mass: 45,
        name: "Iris"
    },
    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) => {
                if (err) console.error(err);
                console.log(animal.name + " the " + animal.color + " " + animal.type + " is a " + animal.size + "-sized animal.");
            });
            db.close(() => {
                console.log("Bağlantıyı kapattım hadi..");
            });
        });
    });        
});

});

This is a Mongoose education and i'm using this locally. I expect to see pre.save method work, but i could not have the result.

Answer 1

it's because you have your bracket in the wrong place after the word save. AnimalSchema.pre("save") it should be

AnimalSchema.pre("save", function (next) {
    if (this.mass >= 100) {
        this.size = "büyük";
    } else if (this.mass >=5 && this.mass < 100) {
        this.size = "orta";
    } else {
        this.size = "küçük";
    }
    next();
});