JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" class="dec" maxlength= "10" id="dec" /><br>
<span id="output"></span>

JavaScript

$(".dec").focusout(function (event) {
    var nondecimalRegex = /^\d{1,6}$/,
    	inputtxt = event.target.value,
        decimalRegex = /^\d{1,6}\.\d{3}$/;
    if (inputtxt.length > 0) {
        var resultVal;
        if (/^\d{1,6}$/.test(inputtxt)) {
            resultVal = inputtxt + ".000";
        } else if (/^\d{1,6}\.\d{1,3}$/.test(inputtxt)) {
            // count of zeros to add to the end of input val
            var c = 3 - inputtxt
                .split("\.")[1].length;
            resultVal = inputtxt 
                + (c == 1 ? "0" : (c == 2 ? "00" : ""));
        }
        dec.value = typeof resultVal == "undefined" 
            ? "" : resultVal;
    }
});
/*$(".dec").focusin(function (event) {
    output.innerHTML = "awd";
});*/