Adding a sub-schema to use in place of a Mixed type...

Product.js

import mongoose from 'mongoose';

const Schema = mongoose.Schema;

const schemaOptions = {
  timestamps: true
};

// this defines a sub-schema that will be used inside the Product schema...
const variantSchema = new Schema({
  qty: { type: Number, required: true },
  price: {
    type: Number,
    required: true
  },
  originalPrice: {
    required: false,
    type: Number
  },
  size: { type: String }
});

const productSchema =  new Schema({

  productTitle: {
    type: String,
    required: true,
    index: true
  },
  slug: {
    type: String
  },
  qty: { type: Number, required: true },
  desc: {
    type: String,
    required: true,
    index: true
  },
  category: {
    type: String,
    required: true
  },
  price: {
    type: Number,
    required: true
  },
  originalPrice: {
    required: false,
    type: Number
  },
  
  // using the variantSchema to define an array of variants
  variants: [{ type: variantSchema, excludeIndexes: true }],
  
//  productOptions: {
//    type: Schema.Types.Mixed
//  },
  
  defaultImg: {
    type: String,
    default: ''
  },
  publish: String,
  freeShipping: String,
  userId: {
    type: Schema.Types.ObjectId,
    ref: 'User',
    required: true
  },
  listingType: {type: String, default: 'product'}


}, schemaOptions);

productSchema.index({productTitle: 'text', desc: 'text'});

module.exports = mongoose.model('Product', productSchema);