JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id = 'yyyy' onkeyup = 'check(this.value)'></textarea>
        <textarea id = 'xxxx' onchange = 'check2(this.value)' readonly></textarea>

JavaScript

//Is not changable according to OP
            function check(value){
                document.getElementById('xxxx').value = value
            };

            //Is not changable according to OP
            function check2(value){
                alert(value)
            };

            window.onload = function(){
                var tE = document.querySelector('#xxxx'); //The readonly textarea

                //Redefine the value property for the textarea
                tE._value = tE.value;
                Object.defineProperty(tE, 'value', {
                    get: function(){return this._value},
                    set: function(v){
                        this._value = v;
                        this.innerHTML = v;
                        this.setAttribute('value', v); //Setting the attribute (browser stuff)
                        check2(v)
                    }
                })
            }