Question

I think I might fail at Googling, and maybe this pattern just doesn’t fit with the way JavaScript handles MRO, but I’m looking for an equivalent to Perl’s AUTOLOAD method such that:

function car() {
  return {
    start: function() { alert('vrooom') },
    catchall: function() { alert('car does not do that'); }
  }
};

car().start(); //vrooom
car().growBeard(); //car does not do that

in Perl to quickly handle this situation I’d write:

sub AUTOLOAD { __PACKAGE__." doesn't do that" }

but the syntax to catch an undefined method in JavaScript eludes me.
Perhaps overloading .call or .apply or something?

Answer 1

If you simply want to know if a method is defined you can do the following:

if(car().growBeard){
}

It sounds like what you are looking for is __noSuchMethod__, however it is only supported in mozilla, and not part of the formal ECMAScript specifications.

function car() {
  return {
    start: function() { alert('vrooom'); },
    __noSuchMethod__ : function (){alert('no method');}
  };
}

Example of __noSuchMethod__ on jsfiddle, only works in a mozilla based browser.

Using simple exception handling you might be able to get your desired behavior:

try {
    car().start(); //vrooom
    car().growBeard();
}
catch (e) {
    if (e instanceof TypeError) {
        alert(e.message);
    }
}