function selectElementContents(el) {

    var range = document.createRange();

    range.selectNodeContents(el);

    var sel = window.getSelection();

    sel.removeAllRanges();

    sel.addRange(range);

}

​

var p = document.querySelector( 'p' );

​

selectElementContents(p);

​

// var a = document.execCommand( 'copy' );

​

​

​

document.querySelector('button').addEventListener('click', function() {

  var b = document.execCommand( 'paste' );

​

  console.log( b );

});