Function to mixin properties including getters and setters from one javascript object to another.

addMixin.js

function mixin(target, ...sources) {

    sources.forEach(source => {
    
        let descriptors = Object.keys(source).reduce((descriptors, key) => {
            descriptors[key] = Object.getOwnPropertyDescriptor(source, key);
            return descriptors;
        }, {});
      
        Object.getOwnPropertySymbols(source).forEach(sym => {
            let descriptor = Object.getOwnPropertyDescriptor(source, sym);
            if(descriptor.enumerable) {
                descriptors[sym] = descriptor;
            }
        });
        
        Object.defineProperties(target, descriptors);
        
    });
   
    return target;
}

All credit to @calebporzio for the mixin function. He has explained it in his blog .