Change Variables Depending on Selected Set of Checkbox

I have a set of checkbox which has different values depending on the combination of selected boxes, but the code I made didn't work, I was thinking maybe the location of my && sign is not in proper position but I don't know where exactly I can put it to run the code.

Here is the html code:

<form role="form" id="checkbox" name="checkbox" onclick="changevalue()">
    <input type="checkbox" id="f1">Full Front
    <input type="checkbox" id="b1">Full Back
    <input type="checkbox" id="f2">Pocket
</form>

<input type="number" id="checkboxResult">

And here is the script:

 <script type="text/javascript">
    function changevalue(form){
        var chkboxvalue = 0;
            if (document.getElementById('f1').checked) &&
            if (document.getElementById('b1').checked) {
                chkboxvalue = 2.2;
            };

            if (document.getElementById('f2').checked) &&
            if (document.getElementById('b1').checked) {
                chkboxvalue = 1.7;
            };

        document.getElementById('checkboxResult').value = chkboxvalue;
    }
</script>