Knockout Js Input Box Event Change Passes Old Value

Have you ever come across a situation where you needed to pass the old value of an input field in a Knockout.js application when the value changes? Well, fret not, because in this article, we'll discuss a simple and effective way to achieve this using Knockout.js.

Knockout.js is a powerful JavaScript library that simplifies the dynamic creation of responsive user interfaces. One common requirement in web applications is to track changes in input fields and perform certain actions based on the old and new values. By default, Knockout.js doesn't provide a direct way to access the old value of an input field when it changes. However, with a little workaround, we can easily accomplish this task.

To pass the old value of an input field when it changes in Knockout.js, we can leverage the `subscribe` function available in Knockout observables. Observables in Knockout.js are key to achieving data binding and reactivity in applications. By subscribing to changes in an observable, we can capture both the old and new values when the input field is updated.

Here's a step-by-step guide on how to implement this functionality:

1. Define an observable to store the current value of the input field:

let inputValue = ko.observable('');

2. Create a computed observable to track changes in the input field:

let oldInputValue;
let trackedInputValue = ko.computed({
    read: function () {
        return inputValue();
    },
    write: function (newValue) {
        oldInputValue = inputValue();
        inputValue(newValue);
    }
});

// Subscribe to changes in the input field
trackedInputValue.subscribe(function (newValue) {
    console.log('Old Value:', oldInputValue);
    console.log('New Value:', newValue);
});

In the above code snippet, we first define the observable `inputValue` to store the current value of the input field. Next, we create a computed observable `trackedInputValue` that reads and writes the input field's value while also capturing the old value before updating it. Finally, we subscribe to changes in `trackedInputValue` and log both the old and new values to the console.

By following these steps, you can effectively pass the old value of an input field when it changes in a Knockout.js application. This approach enables you to track changes in input fields and take necessary actions based on the old and new values.

In summary, leveraging Knockout observables and computed observables, along with the `subscribe` function, allows you to efficiently handle input field value changes and access the old value when needed. This technique enhances the interactivity and responsiveness of your web applications built using Knockout.js. So go ahead, implement this solution in your projects, and make your user interfaces even more dynamic and engaging!