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

The accepted answer is correct. Just wanted to add one more option. You also can use try ... catch block to handle this situation. A freaky example:

var a;
try {
    a = b + 1;  // throws ReferenceError if b is not defined
} 
catch (e) {
    a = 1;      // apply some default behavior in case of error
}
finally {
    a = a || 0; // normalize the result in any case
}

Be aware of catch block, which is a bit messy, as it creates a block-level scope. And, of course, the example is extremely simplified to answer the asked question, it does not cover best practices in error handling ;).