js如何捕获input文本框内容改变事件

<input type="text" id="testId" readonly onPropertyChange="showValue(this.value)" />
<script>
function dochg(){
document.getElementById("testId").value = "test"
}
setTimeout("dochg()",1000)
function showValue(obj){
alert("changed value is " + obj);
}
</script>