javascript: define a variable if it doesn't exist

Pro style:

var SomeVar = SomeVar || 'Default Value';

if (typeof variable === 'undefined') {
    // variable is undefined
    // eg:
    // var variable = "someValue";
}

It would be a good coding practice in this case to use the ternary operator. Also you don't need to have three equal signs when comparing with typeof. This is the most concise solution:

b = typeof(b) == 'undefined' ? 0 : b;

This hopefully will save your hands some time.