复制到剪贴板功能

使用execCommand复制到剪贴板 必须先有input之类的元素 可以focus,select，所以思路是先创建了一个textarea元素 把需要复制的内容放进textarea 把边框等都设成0，调用execCommand，然后remove textarea。

    copyToClipboard(text) {
      var textArea = document.createElement("textarea");
      //
      // *** This styling is an extra step which is likely not required. ***
      //
      // Why is it here? To ensure:
      // 1. the element is able to have focus and selection.
      // 2. if element was to flash render it has minimal visual impact.
      // 3. less flakyness with selection and copying which **might** occur if
      //    the textarea element is not visible.
      //
      // The likelihood is the element won't even render, not even a flash,
      // so some of these are just precautions. However in IE the element
      // is visible whilst the popup box asking the user for permission for
      // the web page to copy to the clipboard.
      //

      // Place in top-left corner of screen regardless of scroll position.
      textArea.style.position = 'fixed';
      textArea.style.top = 0;
      textArea.style.left = 0;

      // Ensure it has a small width and height. Setting to 1px / 1em
      // doesn't work as this gives a negative w/h on some browsers.
      textArea.style.width = '2em';
      textArea.style.height = '2em';

      // We don't need padding, reducing the size if it does flash render.
      textArea.style.padding = 0;

      // Clean up any borders.
      textArea.style.border = 'none';
      textArea.style.outline = 'none';
      textArea.style.boxShadow = 'none';

      // Avoid flash of white box if rendered for any reason.
      textArea.style.background = 'transparent';

      textArea.value = text;

      document.body.appendChild(textArea);
      textArea.focus();
      textArea.select();

      try {
        var successful = document.execCommand('copy');
        var msg = successful ? 'successful' : 'unsuccessful';
        console.log('Copying text command was ' + msg);
      } catch (err) {
        console.log('Oops, unable to copy');
      }

      document.body.removeChild(textArea);
    }