JS: Uncaught TypeError: object is not a function (onclick)

Since the behavior is kind of strange, I have done some testing on the behavior, and here's my result:

TL;DR

If you are:

• In a form, and
• uses onclick="xxx()" on an element
• don't add id="xxx" or name="xxx" to that element
  • (e.g. <form><button id="totalbandwidth" onclick="totalbandwidth()">BAD</button></form> )

Here's are some test and their result:

Control sample (can successfully call function)

function totalbandwidth(){ alert("Total Bandwidth > 9000Mbps"); }

<form onsubmit="return false;">
  <button onclick="totalbandwidth()">SUCCESS</button>
</form>

Add id to button (failed to call function)

function totalbandwidth(){ alert("Total Bandwidth > 9000Mbps"); }

<form onsubmit="return false;">
  <button id="totalbandwidth" onclick="totalbandwidth()">FAILED</button>
</form>

Add name to button (failed to call function)

function totalbandwidth(){ alert("Total Bandwidth > 9000Mbps"); }

<form onsubmit="return false;">
  <button name="totalbandwidth" onclick="totalbandwidth()">FAILED</button>
</form>

Add value to button (can successfully call function)

function totalbandwidth(){ alert("Total Bandwidth > 9000Mbps"); }

<form onsubmit="return false;">
  <input type="button" value="totalbandwidth" onclick="totalbandwidth()" />SUCCESS
</form>

Add id to button, but not in a form (can successfully call function)

function totalbandwidth(){ alert("Total Bandwidth > 9000Mbps"); }

<button id="totalbandwidth" onclick="totalbandwidth()">SUCCESS</button>

Add id to another element inside the form (can successfully call function)

function totalbandwidth(){ alert("The answer is no, the span will not affect button"); }

<form onsubmit="return false;">
<span name="totalbandwidth" >Will this span affect button? </span>
<button onclick="totalbandwidth()">SUCCESS</button>
</form>

Please change only the name of the function; no other change is required

<script>
    function totalbandwidthresult() {
        alert("fdf");
        var fps = Number(document.calculator.fps.value);
        var bitrate = Number(document.calculator.bitrate.value);
        var numberofcameras = Number(document.calculator.numberofcameras.value);
        var encoding = document.calculator.encoding.value;
        if (encoding = "mjpeg") {
            storage = bitrate * fps;
        } else {
            storage = bitrate;
        }

        totalbandwidth = (numberofcameras * storage) / 1000;
        alert(totalbandwidth);
        document.calculator.totalbandwidthresult.value = totalbandwidth;
    }
</script>

<form name="calculator" class="formtable">
    <div class="formrow">
        <label for="rcname">RC Name</label>
        <input type="text" name="rcname">
    </div>
    <div class="formrow">
        <label for="fps">FPS</label>
        <input type="text" name="fps">
    </div>
    <div class="formrow">
        <label for="bitrate">Bitrate</label>
        <input type="text" name="bitrate">
    </div>
    <div class="formrow">
        <label for="numberofcameras">Number of Cameras</label>
        <input type="text" name="numberofcameras">
    </div>
    <div class="formrow">
        <label for="encoding">Encoding</label>
        <select name="encoding" id="encodingoptions">
            <option value="h264">H.264</option>
            <option value="mjpeg">MJPEG</option>
            <option value="mpeg4">MPEG4</option>
        </select>
    </div>Total Storage:
    <input type="text" name="totalstorage">Total Bandwidth:
    <input type="text" name="totalbandwidth">
    <input type="button" value="totalbandwidthresult" onclick="totalbandwidthresult();">
</form>