Question

Is it possible to cancel object creation in coffeescript if certain criteria are not met? I want to do something like this:

class Foo
  constructor:(@name) ->
    return false if !name

withName = new Foo("bar") #withName -> obj
noName = new Foo #noName -> false

But with that code the object is still created. What is the best way to do this?

Answer 1

You need to check the validation conditions at the call site, inside the constructor is far to late.

Take a look at the compiled javascript for the class:

 Foo = (function() {

    function Foo(name) {
      this.name = name;
      if (!name) return false;
    }

    return Foo;

  })();

  withName = new Foo("bar");

  noName = new Foo;

}).call(this);

And it should be a little more clear why it doesn’t work.

The new keyword happens before the constructor is called.

How about something like this (NOT TESTED)

runIf = (someVar, someDelegate) ->
    someDelegate someVar if someVar 

withName = runIf "bar", -> new Foo("bar") 
noName = runIf null, -> new Foo() 

Which compiles down to:

runIf = function(someVar, someDelegate) {
    if (someVar) return someDelegate(someVar);
  };

  withName = runIf("bar", function() {
    return new Foo("bar");
  });

  noName = runIf(null, function() {
    return new Foo();
  });