For a simple example of the onchange event hander here is a simple example that uses document.querySelector to get references to an input tag, and a paragraph tag. When the text of the text input element changes the event fires, and the value of the input element can be used to update the output that is set in the paragraph element.
When adding an event listener for the onchange event with addEventListener the first argument is the event that I want to attach for, in this case it is onchange. The second argument is a call back method that will fire each time this event occurs for the input element. the first argument that is suppled to this callback method is an event object that can be used to gain the value of the element from which the event fired.
In this event object I am using the target property of the event object as a way to gain a reference to the input element as well. In this example it is not a big deal to just use the inputPow global, however in a more complex project that involves a lot of elements it is often better to use the target property inside the body of a callback method as a way to gain access to the element from which an event like onchange as fired.
For a more advanced example of the onchange event, it made a quick little app that can be used to estimate the amount of money that a blog can make if the revenue per mille, and page views counts are known. In this example I am attaching the same event handler for more than one input element.
Here I am using Function.call to use the Array.forEach method as a way to loop over the children property of my controls div. This is necessary if I want to use an Array method like Array.forEach with the children property becuase it is not an Array but rather an HTMLCollection.