With document.getElemebtById() we can have access and modify the properties of any HTML element for instance if we have the following element:
<input id="myText" type="text" />
To access all it’s properties like style or value, we just refer to the element and make the change:
<script>
document.getElementById("myText").value = "Hello";
</script>
If you put this pieces of code into a page it will not work because there is no action that call the the assigned variable, to make a proper call we should create a function or place a button if you like: