Am I doing it right ? ;-)

dunno.js

(()=>{ "use strict"

    let mixinHandler = {
            get: (t, n)=>{
                if ( t[n] != void 0 )
                  return t[n]
                 
                for ( let i = t.__parents__.length-1; ~i; i-- )
                  if ( t.__parents__[i][n] != void 0 )
                    return t.__parents__[i][n]
                
                return Object.prototype[n]
            }
          , set: (t, n, v) => t[n] = v
        }
      , proxyfy = parents => Proxy(Object.create(null, { __parents__: { value: parents } }), mixinHandler)
      , mixin = this.mixin = (...args)=> typeof args[args.length-1] == "function" ? args.pop().call(null, proxyfy(args)) : proxyfy(args)

})()

mixin({foo:function(){ return this.bar }}, {bar:"foofoo"}, Array.prototype, p=>{
    p.foobar = "foobar"
    
    p.push("foo", "bar")
    p.sort((a, b)=>a!==b)
    
    console.log(p.foo(), p.foobar, p) // "foofoo", "foobar", { 0: "bar", 1: "foo", foobar: "foobar", length: 2 }
})