creating a a custom mongoose schema type

mongoose-custom-type.js

'use strict';

var assert = require('assert');
var mongoose = require('mongoose');
var Schema = mongoose.Schema;

mongoose.connect('mongodb://localhost/db');
mongoose.set('debug', true);

class customType {
  constructor(v) {
    this.v = v;
  }

  toBSON() {
    return this.v;
  }
}

class customTypeSchema extends mongoose.SchemaType {
  cast(v) {
    return new customType(v);
  }
}

mongoose.Schema.Types.customType = customTypeSchema;

const schema = new Schema({ test: customType });
const testing = mongoose.model('test', schema);

testing.create({ test: 5 }).then(doc => {
  console.log('done', doc);
  console.log(doc.test.toBSON());
  process.exit(0);
});

mongoose.Schema.Types.customType = customTypeSchema;
this line is not working.
Property 'customType' does not exist on type 'typeof Types'.
I'm using typescript.