Synchronous dynamic script loading

[howa]

Hello,

I have the below codes, in which I have a variable "x" declared
test.js,

// =================================

var str = "<scri" + "pt type='text/javascript' src='http://
www.example.com/test.js'></sc" + "ript";

document.write(str);
alert( x );

// =================================


How to make sure alert() is only after the external script is always
loaded?

Thanks.