How to Limit Character Length of Copy from Website and Add ...Read More at "Website Link"

Some websites now use a JavaScript service from Tynt that appends text to copied content. I am using Wordpress and want some Javascript that Prevent users to copy whole content. I want to limit number of characters and put read more link on Copied text. Is there any Javascript Solution. I am currently using this code, but it is not working properly.

    
    var selection = window.getSelection(),
        pagelink = '<br /><br /> Read more at: ' + document.location.href,
        copytext = selection.substring(0,200) + pagelink,
        newdiv = document.createElement('div');
    var range = selection.getRangeAt(0); 

   
    newdiv.style.position = 'absolute';
    newdiv.style.left = '-99999px';

       document.body.appendChild(newdiv);
    newdiv.innerHTML = copytext;
    selection.selectAllChildren(newdiv);

    window.setTimeout(function () {
        document.body.removeChild(newdiv);
        selection.removeAllRanges();
        selection.addRange(range);
    }, 100);
}

document.addEventListener('copy', addLink);

Kindly Modify or provide new code.