How to check data before changing data in vue.js?

Question

Vue.js

Good afternoon, there is an input field, how can I check the input data, if they do not pass validation, then do not change the variable?

let app = new Vue({
        el: '#productsCart',
        data: {
            quantity: 5
        },
//вочер срабатывает после того как пользователь 
        watch: {
            quantity: function (newData, oldData) {
                console.log(oldData)
            }
        },
    })

Answer 1

Well, create a method in it, write validation, if validation passes, change the variable.

Answer 2

Make two objects with data - one connected to the fields through the model, which will reflect their state, and the other validated data to send.
Here's how to keep track of data changes in the object associated with the fields and transfer it to the object for sending when they are valid.