When I've developed APIs like these in the past, I've validated anything that I feel is a "major" requirement - in your example, I'd verify the first two arguments.
As long as you specify sensible defaults, it should be pretty simple for your user to determine that "optional" arguments aren't specified correctly, since it won't make any change to the application, but everything will still work properly.
If the API is complex, I'd suggest following Aaron's advice - add comments that can be parsed by a compressor around your validation so the developers get the benefit of validation, but can extract the extra dead weight when pushing the code into production.
EDIT:
Here're some examples of what I like to do in the cases where validation is necessary. This particular case is pretty simple; I probably wouldn't bother with validation for it, since it really is trivial. Depending on your needs, sometimes attempting to force types would be better than validation, as demonstrated with the integer value.
Assume extend() is a function that merges objects, and the helper functions exist:
var f = function(args){
args = extend({
foo: 1,
bar: function(){},
biz: 'hello'
}, args || {});
// ensure foo is an int.
args.foo = parseInt(args.foo);
//<validation>
if(!isNumeric(args.foo) || args.foo > 10 || args.foo < 0){
throw new Error('foo must be a number between 0 and 10');
}
if(!isFunction(args.bar)){
throw new Error('bar must be a valid function');
}
if(!isString(args.biz) || args.biz.length == 0){
throw new Error('biz must be a string, and cannot be empty');
}
//</validation>
};
EDIT 2:
If you want to avoid common misspellings, you can either 1) accept and re-assign them or 2) validate the argument count. Option 1 is easy, option 2 could be done like this, although I'd definitely refactor it into its own method, something like Object.extendStrict() (example code works w/ prototype):
var args = {
ar: ''
};
var base = {
foo: 1,
bar: function(){},
biz: 'hello'
};
// save the original length
var length = Object.keys(base).length;
// extend
args = Object.extend(base, args || {});
// detect if there're any extras
if(Object.keys(args).length != length){
throw new Error('Invalid argument specified. Please check the options.')
}