If you want to allow undefined to be passed as a valid non-default value for a trailing parameter, you can also use arguments.length to determine how many argum

If you want to allow undefined to be passed as a valid non-default value for a trailing parameter, you can also use arguments.length to determine how many arguments were explicitly specified. var foo = function(a, b) { if (arguments.length < 2) { b = 'some default value'; } // ... };