check if object is instanceof mixin

Question

I'd like to check if the object x implements (is instance of) the mixin MyInterface:

Ext.define('MyInterface', {
    interfaceMethod: Ext.emptyFn
});

Ext.define('Foo', {
    mixins: {
        myInterface: 'MyInterface'
    }
});

var x = new Foo();

doesn't work work:

console.log(x instanceof MyInterface);

ugly workaround:

var isInstanceOfMyInterface = false;
for (var i in x.mixins) {
    if (x.mixins[i].$className == 'MyInterface') {
        isInstanceOfMyInterface = true;
    }
}
console.log(isInstanceOfMyInterface);

The workaround has one major issue: it doesn't work if a subclass of MyInterface is used.

jsfiddle

A pattern used in the core is to add some identifying property on a class or mixin that can be checked. So for example add an `isMyInterface` property on the interface which then gets mixed into `Foo`. — Evan Trimboli, May 30 '14 at 06:33
That's it - thanks - please create an answer for that so I can accept it. — Niko Sams, May 30 '14 at 07:31

score 5 · Accepted Answer · answered May 30 '14 at 08:37

A pattern the Ext core uses is to apply a mixin specific property so you can test for its existence. It's a lot cheaper than an instanceof check as well. For example:

Ext.define('MyInterface', {
    isMyInterface: true,
    interfaceMethod: Ext.emptyFn,
});

Ext.define('Foo', {
    mixins: {
        myInterface: 'MyInterface'
    }
});

var x = new Foo();
console.log(x.isMyInterface);

score 0 · Answer 2 · answered May 30 '14 at 05:42

Mixins solve the problem of multiple inheritance because there is no way to make class C inherit from both A and B in plain JavaScript. The logic is then let class C to inherit from A and use class B as mixin. The mixin B is a sort of repository of additional methods and properties for C.

When the mixin B is used in class C, all its methods and properties are copied to the prototype of C and when C is instantiated it contains its own methods, methods of A, because it inherits from A, and B because mixin's methods are added to it.

Also, C can use more than one mixin to add additional methods/properties. Using mixin does not change the class name of the target class, C stays C.

Internally in Ext used mixins are held as object in mixins object of the target class as name/value pairs.

Summed together, your way is already the fine way to check if a class uses a mixin, nevetheless, it can be improved:

you can implement the above code as a method of the class(es) you want to check
you can break the above look once the mixin is found to save some cycles

While your answer explains mixins it doesn't help for my question :/ — Niko Sams, May 30 '14 at 07:36

check if object is instanceof mixin

doesn't work work:

ugly workaround:

2 Answers2