quicktag.jsからの抜粋

qt.insertContent.js

qt.insertContent = function(content) {
  var sel, startPos, endPos, scrollTop, text, canvas = document.getElementById(wpActiveEditor), event;

  if ( !canvas ) {
    return false;
  }

  if ( document.selection ) { //IE
    canvas.focus();
    sel = document.selection.createRange();
    sel.text = content;
    canvas.focus();
  } else if ( canvas.selectionStart || canvas.selectionStart === 0 ) { // FF, WebKit, Opera
    text = canvas.value;
    startPos = canvas.selectionStart;
    endPos = canvas.selectionEnd;
    scrollTop = canvas.scrollTop;

    canvas.value = text.substring(0, startPos) + content + text.substring(endPos, text.length);

    canvas.selectionStart = startPos + content.length;
    canvas.selectionEnd = startPos + content.length;
    canvas.scrollTop = scrollTop;
    canvas.focus();
  } else {
    canvas.value += content;
    canvas.focus();
  }

  if ( document.createEvent ) {
    event = document.createEvent( 'HTMLEvents' );
    event.initEvent( 'change', false, true );
    canvas.dispatchEvent( event );
  } else if ( canvas.fireEvent ) {
    canvas.fireEvent( 'onchange' );
  }

  return true;
};