How to check a not-defined variable in JavaScript

Question

I wanted to check whether the variable is defined or not. For example, the following throws a not-defined error

alert( x );

How can I cat

Answer 1

You can also use the ternary conditional-operator:

var a = "hallo world";
var a = !a ? document.write("i dont know 'a'") : document.write("a = " + a);

//var a = "hallo world";
var a = !a ? document.write("i dont know 'a'") : document.write("a = " + a);