现在 execCommand(copy) 已过时，将文本复制到剪贴板

// In this example, the text to copy would be in an element with id = textcopy

var text_to_copy = document.getElementById("textcopy").innerHTML;

if (!navigator.clipboard){
    // use old commandExec() way
} else{
    navigator.clipboard.writeText(text_to_copy).then(
        function(){
            alert("yeah!"); // success 
        })
      .catch(
         function() {
            alert("err"); // error
      });
}