JSFiddle - React, Tailwind, and code Playground

HTML

<form name="myForm" id="myForm">
    <input type="text" name="formitemquantity[]" />
    <input type="text" name="formitemquantity[]" />
    <input type="text" name="formitemquantity[]" />
    <input type="text" name="formitemquantity[]" />
    <input type="text" name="formitemquantity[]" />
</form>

JavaScript

var form = document.getElementById("myForm"),
    incInput = form["formitemquantity[]"],
    len = incInput.length,
    ctr = 0;
for(;ctr < len; ctr++){
    incInput[ctr].value = ctr;
}