how to set mongoose schema for select option list and set property on selected option

Mongoose doesn't have the select type and you need to manage the drop-down in you view, but you should define a set of the property so that only those properties will be accepted by that field, by ENUM

const mongoose = require('mongoose')

const Schema = mongoose.Schema()

const productSchema = new Schema(
    {
        name : {
            type: String,
            required : true
        },
        price : {
            type : Number,
            required : true 
        },
        units: {
            type: String,
            enum: ['KG', 'liters', 'meters', 'cm'],
            required : true 
        }
    }
)