How to get the height of the text inside of a textarea

element.scrollHeight is probably worth investigating.

If I was going to approach this (and I've not tested this at all), I'd set the textarea's height to 1px measure the scroll height and then reset the textarea's height.

https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollHeight

Create a span element, set Span's innerHTML to "Hello World".
Get the span's offsetHeight.

var span = document.createElement("span");
span.innerHTML="Hello World"; //or whatever string you want.
span.offsetHeight // this is the answer

note that you must set the span's font style to the textarea's font style.

Your example will NEVER work because innerHTML and value are both strings. String doesn't define offsetWidth.

If you wish to get the height of selected text inside of a textarea, use selectionStart/selectionEnd to find the selected text of the textarea.