why does a variable in javascript seem to hold the value "undefined" even when it was defined

Question

if I run the following javascript code, the browser alerts "undefinedabc".

var x;
x += "abc";
alert(x);

To me it looked like I did define the variable x, so why does it seem to be undefined?

Answer 1

undefined is the default value of any variable with no value assigned. So, var x; means that a = undefined. When you add "abc" to it, you're actually doing undefined + "abc". Finally, undefined is stringifiend to "undefined" and then concatenated to "abc" and turns to "undefinedabc".

In order to concat initialize the var x, you should assign an empty string to it (remember that JavaScript dinamically-typed):

var x = '';
x += "abc";
alert(x);    // "abc"

This MDN article describes this behaviour.