What causes the ReferenceError: Schema is not defined error?

Question

Node.js

var crypto = require("crypto");
var mongoose = require("../libs/mongoose");

var schema = new Schema({
    username:{
        type:Stryng,
        unique:true,
        required:true
    },
    hashedPassword:{
        type:String,
        required:true
    },
    salt:{
        type:String,
        required:true
    },
    created:{
        type:Date,
        default:Date.now
    }
});
schema.metods.encryptPassword = function(password){
    return crypto.createHmac("sha1",this.salt).update(password).digest('hex');
};

schema.virtual('password')
    .set(function(password){
    this._plainPassword = password;
    this.salt = Math.random();
    this.hashedPassword = this.encryptPassword(password);
    })
    .get(function(){return this._plainPassword});
shema.methods.checkPassword = function (password) {
    return this.encryptPassword(password) === this.hashedPassword;
};
exports.User = mongoose.model('User',schema);

this is the error why?

/var/www/html/chatnode/models/user.js:4
var schema = new Schema({
                 ^

ReferenceError: Schema is not defined
    at Object.<anonymous> (/var/www/html/chatnode/models/user.js:4:18)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:456:32)
    at tryModuleLoad (module.js:415:12)
    at Function.Module._load (module.js:407:3)
    at Module.require (module.js:466:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/var/www/html/chatnode/createDb.js:1:74)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:456:32)
    at tryModuleLoad (module.js:415:12)
    at Function.Module._load (module.js:407:3)
    at Function.Module.runMain (module.js:575:10)
    at startup (node.js:159:18)

here is the error

Answer 1

Because you don't have a Schema variable defined,
add to the beginning of the file: var Schema = mongoose.Schema
ps Well, just in case, let's figure it out.
in translation - Schema - not defined. And indeed, if you look at your code, you don't have a style entry anywhere var Schema = ..., so you can't use this variable because it simply doesn't exist. It doesn't require it in your file, you don't define it in any way...