JSFiddle - React, Tailwind, and code Playground

HTML

<input id="amount" type="text" value="123"/>
<input type="button" onclick="FormatNumber()" value="Format"/>
<div id="outputLeadingZeros"></div>
<div id="outputLeadingTwoDecimal"></div>

<script>
function FormatNumber ()
{
    var amount = parseFloat(document.getElementById('amount').value);
    
    document.getElementById('outputLeadingZeros').innerHTML =
        amount.toLocaleString('en-US',
        {
            style: 'decimal',
            minimumIntegerDigits: 6,
            useGrouping: false
        });
    
    document.getElementById('outputLeadingTwoDecimal').innerHTML =
        amount.toLocaleString('en-US',
        {
            style: 'decimal',
            minimumFractionDigits: 2,
            useGrouping: false
        });
}
</script>