Обнаружение изменения входного значения с помощью MutationObserver

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;
            }
        });
    }
}

<input type="text" id="inputBox" placeholder="Enter something" />

function changeValue (event, target) {
    document.querySelector("#" + target).value = new Date().getTime();
}
 
function changeContentValue () {
    document.querySelector("#content").value = new Date().getTime();
}
 
Object.defineProperty(document.querySelector("#content"), "value", {
    set:  function (t) {
        alert('#changed content value');
        var caller = arguments.callee
            ? (arguments.callee.caller ? arguments.callee.caller : arguments.callee)
            : ''
 
        console.log('this =>', this);
        console.log('event => ', event || window.event);
        console.log('caller => ', caller);
        return this.textContent = t;
    }
});

<form id="form" name="form" action="test.php" method="post">
        <input id="writer" type="text" name="writer" value="" placeholder="writer" /> <br />
        <textarea id="content" name="content" placeholder="content" ></textarea> <br />
        <button type="button" >Submit (no action)</button>
</form>
<button type="button" onClick="changeValue(this, 'content')">Change Content</button>