As its name implies, javascript is a script-based language. That means nothing is pre-compiled and arguments are passed around as generic variants (var)s. These variants could be a literal value such as a numeric 1,2,3... or could be a reference
to an object that exists on an HTML page, or that was created in script (object reference). Javascript tries to interpret the argument type according to what the function is trying to do with it. Thus the function window.alert(1); and
window.alert("1"); produce the same result, since the script engine chooses to interpret both as a string value. The function window.alert(1+"1"); results in the alert message "11" while the function window.alert(1+1) results in the alert message "2"
because the script engine interprets the values as numeric, adds them up, then interprets the result as a string and displays that.
http://www.w3schools.com/js/
-JayB
