How do I copy to the clipboard in JavaScript?

html page: 

 <button onclick="copyToClipboard();return false">copy to the clipboard</button> 
 <asp:HiddenField runat="server" ID="hdnCopyContent" ClientIDMode="Static" value="clipboard in JavaScript" />

Javascript function

function copyToClipboard() {     
        var text = $("input[Id$='hdnCopyContent']").val();
        if (text != undefined || text != null || text != "") {       
            alert(text);
        }       
        if (window.clipboardData && window.clipboardData.setData) {
            // IE specific code path to prevent textarea being shown while dialog is visible.
            return clipboardData.setData("Text", text);

        } else if (document.queryCommandSupported && document.queryCommandSupported("copy")) {
            var textarea = document.createElement("textarea");
            textarea.textContent = text;
            textarea.style.position = "fixed";  // Prevent scrolling to bottom of page in MS Edge.
            document.body.appendChild(textarea);
            textarea.select();
            try {
                return document.execCommand("copy");  // Security exception may be thrown by some browsers.
            } catch (ex) {
                console.warn("Copy to clipboard failed.", ex);
                return false;
            } finally {
                document.body.removeChild(textarea);
            }
        }
    }