TypeError when trying to Proxy a ES6 class constructor

问题

I'm trying to Proxy a ES6 constructor (mostly trying to emulate Python's descriptors for fun and learning):

class _Record {
    constructor(data) {
        this._data = data;
    }
}

const constrProxyHandlers = {
    construct: function(target, args, cstr) {
        let instProxyHandler = {
            get: (target, prop) => {
                if(prop in target._data) { return target._data[prop] }
                return target[prop]
            },
            set: (target, prop, val) => {
                if(prop in target._data) {
                    target._data[prop] = val;
                } else {
                    target[prop] = val;
                }
                return true
            }
        }
        
        let obj = Object.create(cstr.prototype)
        cstr.apply(obj, args)
        return new Proxy(obj, instProxyHandler)
    }
}

const Record = new Proxy(_Record, constrProxyHandlers)

let rec = new Record({name: 'Vic', age: 25})
console.log([rec.name, rec.age])
rec.name = 'Viktor'
console.log(rec)

If you run this snippet, you'll get this error:

cstr.apply(obj, args)

TypeError: Class constructor _Record cannot be invoked without 'new'

If I replace cstr.apply with new cstr, the constructor quickly exhausts stack (apparently going into an infinite recursion).

This works if I replace the _Record class with a function (for example, this would work transpiled through Babel). Can I make it work with native ES6?

Thank you.

P. S. I'm currently checking these snippets on Node 7.7.4, if it matters.

回答1:

The part

let obj = Object.create(cstr.prototype)
cstr.apply(obj, args)

does not work with ES6 classes. You need to use

let obj = Reflect.construct(target, args, cstr);

(and not Reflect.construct(cstr, args) or new cstr(...args) that you were trying to do, which indeed recurses infinitely - IIRC target refers to _Record while cstr refers to Record or one of its subclasses).

来源：https://stackoverflow.com/questions/43104420/typeerror-when-trying-to-proxy-a-es6-class-constructor

标签

javascript

node.js

ecmascript-6

es6-proxy