why string.length return undefined?

Question

In the popup window, selText does have the value "great," but the length is always undefined. Something related with the encoding of the string?

var selText = document.getSelection(); //suppose "great" is selected
alert( "selected ->" + selText + " len is " + selText.length);

Answer 1

Because you're getting a DOM selection object instead of a String. To get the text, call toString().

var selText = document.getSelection().toString();

The reason the string successfully shows up in the alert, is that the concatenation causes an implicit toString() to occur.

Answer 2

The MDN documentation states.

In the above example, selObj is automatically "converted" when passed to window.alert. However, to use a JavaScript String property or method such as length or substr, you must manually call the toString method.
-- https://developer.mozilla.org/en/window.getSelection

It's suggesting you call document.getSelection().ToString().length;