What is innerHTML on input elements?

Setting the value is normally used for input/form elements. innerHTML is normally used for div, span, td and similar elements.

value applies only to objects that have the value attribute (normally, form controls).

innerHtml applies to every object that can contain HTML (divs, spans, but many other and also form controls).

They are not equivalent or replaceable. Depends on what you are trying to achieve

First understand where to use what.

<input type="text" value="23" id="age">

Here now

var ageElem=document.getElementById('age');

So on this ageElem you can have that many things what that element contains.So you can use its value,type etc attributes. But cannot use innerHTML because we don't write anything between input tag

  <button id='ageButton'>Display Age</button>

So here Display Age is the innerHTML content as it is written inside HTML tag button.