在输入元素上的javascript更改事件只会导致焦点丢失

I have an input element and I want to keep checking the length of the contents and whenever the length becomes equal to a particular size, I want to enable the submit button, but I am facing a problem with the onchange event of javascript as the event fires only when the input element goes out of scope and not when the contents change.I have an input element and I want to keep chec