JSFiddle - React, Tailwind, and code Playground

JavaScript

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

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

    var x = new Foo();

    //doesn't 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);