detect value change in input tag with vanilla javascript and MutationObserver

Question

I want to detect when text/value change in input field. Even if I change the value with js, I want to detect that changes.

Here\'s what I\'ve tried so far in demo in

Answer 1

I've modified Shawn's method a little and wanted to share it. Can't believe there's actually a solution to this.

Type into the input box to see the default behavior. Now, open the DevTools and select the input element, then change its value, e.g. $0.value = "hello". Examine the UI vs. API difference. It seems UI interactions do not modify value property directly. If it were, it would also log "...changed via API...".

let inputBox = document.querySelector("#inputBox");

inputBox.addEventListener("input", function () {
    console.log("Input value changed via UI. New value: '%s'", this.value);
});

observeElement(inputBox, "value", function (oldValue, newValue) {
    console.log("Input value changed via API. Value changed from '%s' to '%s'", oldValue, newValue);
});

function observeElement(element, property, callback, delay = 0) {
    let elementPrototype = Object.getPrototypeOf(element);
    if (elementPrototype.hasOwnProperty(property)) {
        let descriptor = Object.getOwnPropertyDescriptor(elementPrototype, property);
        Object.defineProperty(element, property, {
            get: function() {
                return descriptor.get.apply(this, arguments);
            },
            set: function () {
                let oldValue = this[property];
                descriptor.set.apply(this, arguments);
                let newValue = this[property];
                if (typeof callback == "function") {
                    setTimeout(callback.bind(this, oldValue, newValue), delay);
                }
                return newValue;
            }
        });
    }
}