if not defined javascript

1var a;
2try {
3    a = b + 1;  // throws ReferenceError if b is not defined
4} 
5catch (e) {
6    a = 1;      // apply some default behavior in case of error
7}
8finally {
9    a = a || 0; // normalize the result in any case
10}
11

1/* Means something was not defined for example you wanted to print */ "X"
2/* but X is not defined to defined you have to make a variable for it */
3X = "Something" /* Once you define it it'll print the value successfully */