Auto Formatting Currency With Jquery

Auto format currency input field with commas and decimals if needed. Text is automatically formated with commas and cursor is placed back where user left off after formatting vs cursor moving to the end of the input. Validation is on KeyUp and a final validation is done on blur.

HTML

<input type="text" class="currency">

Javascript

$(".currency").on({
    keyup: function() {
        let input_val = $(this).val();
        input_val = numberToCurrency(input_val);
        $(this).val(input_val);
    },
    blur: function() { 
        let input_val = $(this).val();
        input_val = numberToCurrency(input_val, true, true);
        $(this).val(input_val);
    }
});

var numberToCurrency = function (input_val, fixed = false, blur = false) {
    // don't validate empty input
    if(!input_val) {
        return "";
    }
    
    if(blur) {
        if (input_val === "" || input_val == 0) { return 0; }
    }

    if(input_val.length == 1) {
        return parseInt(input_val);
    }

    input_val = ''+input_val;
    
    let negative = '';
    if(input_val.substr(0, 1) == '-'){
        negative = '-';
    }
    // check for decimal
    if (input_val.indexOf(".") >= 0) {
        // get position of first decimal
        // this prevents multiple decimals from
        // being entered
        var decimal_pos = input_val.indexOf(".");

        // split number by decimal point
        var left_side = input_val.substring(0, decimal_pos);
        var right_side = input_val.substring(decimal_pos);

        // add commas to left side of number
        left_side = left_side.replace(/\D/g, "").replace(/\B(?=(\d{3})+(?!\d))/g, ",");

        if(fixed && right_side.length > 3) {
            right_side = parseFloat(0+right_side).toFixed(2);
            right_side =  right_side.substr(1, right_side.length);
        }

        // validate right side
        right_side = right_side.replace(/\D/g, "").replace(/\B(?=(\d{3})+(?!\d))/g, ",");

        // Limit decimal to only 2 digits
        if(right_side.length > 2) {
            right_side = right_side.substring(0, 2);
        }
    
        if(blur && parseInt(right_side) == 0) {
            right_side = '';
        }

        // join number by .
        // input_val = left_side + "." + right_side;

        if(blur && right_side.length < 1) {
            input_val = left_side;
        } else {
            input_val = left_side + "." + right_side;
        }
    } else {
        // no decimal entered
        // add commas to number
        // remove all non-digits
        input_val = input_val.replace(/\D/g, "").replace(/\B(?=(\d{3})+(?!\d))/g, ",");
    }

    if(input_val.length > 1 && input_val.substr(0, 1) == '0' && input_val.substr(0, 2) != '0.' ) {
        input_val = input_val.substr(1, input_val.length);
    } else if(input_val.substr(0, 2) == '0,') {
        input_val = input_val.substr(2, input_val.length);
    }
    
    return negative+input_val;
};
Demo
<!DOCTYPE html>
<html lang="en">
<head>
    <title>Bfotool Demo</title>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">

    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
    <div class="container">
        <div class="row">
            <div class="col-md-12">
                <br><br>
                <div class="form-group">
                    <label for="usr">Enter Amount:</label>
                    <input type="text" class="form-control currency">
                </div>
            </div>
        </div>
    </div>

    <script>
        $(".currency").on({
            keyup: function() {
                let input_val = $(this).val();
                input_val = numberToCurrency(input_val);
                $(this).val(input_val);
            },
            blur: function() { 
                let input_val = $(this).val();
                input_val = numberToCurrency(input_val, true, true);
                $(this).val(input_val);
            }
        });

        var numberToCurrency = function (input_val, fixed = false, blur = false) {
            // don't validate empty input
            if(!input_val) {
                return "";
            }
            
            if(blur) {
                if (input_val === "" || input_val == 0) { return 0; }
            }
        
            if(input_val.length == 1) {
                return parseInt(input_val);
            }

            input_val = ''+input_val;
            
            let negative = '';
            if(input_val.substr(0, 1) == '-'){
                negative = '-';
            }
            // check for decimal
            if (input_val.indexOf(".") >= 0) {
                // get position of first decimal
                // this prevents multiple decimals from
                // being entered
                var decimal_pos = input_val.indexOf(".");

                // split number by decimal point
                var left_side = input_val.substring(0, decimal_pos);
                var right_side = input_val.substring(decimal_pos);

                // add commas to left side of number
                left_side = left_side.replace(/\D/g, "").replace(/\B(?=(\d{3})+(?!\d))/g, ",");

                if(fixed && right_side.length > 3) {
                    right_side = parseFloat(0+right_side).toFixed(2);
                    right_side =  right_side.substr(1, right_side.length);
                }

                // validate right side
                right_side = right_side.replace(/\D/g, "").replace(/\B(?=(\d{3})+(?!\d))/g, ",");

                // Limit decimal to only 2 digits
                if(right_side.length > 2) {
                    right_side = right_side.substring(0, 2);
                }
            
                if(blur && parseInt(right_side) == 0) {
                    right_side = '';
                }

                // join number by .
                // input_val = left_side + "." + right_side;

                if(blur && right_side.length < 1) {
                    input_val = left_side;
                } else {
                    input_val = left_side + "." + right_side;
                }
            } else {
                // no decimal entered
                // add commas to number
                // remove all non-digits
                input_val = input_val.replace(/\D/g, "").replace(/\B(?=(\d{3})+(?!\d))/g, ",");
            }

            if(input_val.length > 1 && input_val.substr(0, 1) == '0' && input_val.substr(0, 2) != '0.' ) {
                input_val = input_val.substr(1, input_val.length);
            } else if(input_val.substr(0, 2) == '0,') {
                input_val = input_val.substr(2, input_val.length);
            }
            
            return negative+input_val;
        };
    </script>
</body>
</html>