Javascript从另一个不在移动设备上工作的字段设置隐藏字段值

我有一个隐藏字段,它应该从div中获取字符串值,在浏览器中工作时一切都很好,但在平板电脑或智能手机上运行时却不起作用。我不明白。

这是我的div和隐藏字段:

div id="signature-div" style="border: dotted 2px grey;">
    <div id="canvas"></div>
</div>

<input name="signature" type="hidden" id="signature" value="">

以及功能:

$(document).ready(function() {

    // signature
    var W = $("#signature-div").width();
    var sigCanvas = $("#canvas").jSignature({width: W, height: 180, "background-color":"#ddd"});

    // after signing the offer set hidden field value to signature
    $(document).on('mouseup', '#canvas',function(){
    //$("form").submit(function() {
        var rawSig = $("#canvas").jSignature("getData","svg");
        //$("#img").attr("data", 'data:' + rawSig);

        //$("#signature").val('data:' + rawSig);
        document.getElementById("signature").value = 'data:' + rawSig;

        // i have tried both of these up and it doesn't set the hidden value on mobile devices...is there something i'm missing here?

    });

});