JSFiddle - React, Tailwind, and code Playground
HTML
<input type="text" name="input[]" id="input0" class="input" value='3' >
JavaScript
var somevar=15;
$('#input0').val( parseFloat($('#input0').val()) + somevar );
<input type="text" name="input[]" id="input0" class="input" value='3' >
var somevar=15;
$('#input0').val( parseFloat($('#input0').val()) + somevar );