javascript - JS 代理拦截键和嵌套键不返回回退值

Question

我有一个对象，我想拦截对其任何属性的访问，如果它不存在则返回后备值。我尝试了以下实现，但由于某种原因，我返回了一个 Proxy 对象而不是后备值。

const isObject = obj => typeof obj === 'object';
const hasKey = (obj, key) => key in obj;
const isNullOrUndefined = obj => obj === null || obj === undefined;

const obj = {
  keyOne: ''
};


const returnPropertyOnObject = (target, property) => {
  if (isObject(target[property])) {
    return safe(target[property])
  }

  return target[property];
}

function safe(obj) {
  return new Proxy(obj, {
    get: (target, property) => {

      if (hasKey(target, property) && !isNullOrUndefined(target[property])) {
        returnPropertyOnObject(target, property)
      }

      return new Proxy({}, {
        get: function(target, property) {
          return 'MISSING!!';
        }
      });;
    }
  });
}

const wrap = safe(obj);

console.log(wrap.notExists);
console.log(wrap.notExists.deep.nested.nested);

此外，我有时会在代理内部获取诸如 toJSON 或 toString 之类的属性。

我错过了什么？

Answer 1

一旦您从内部代理返回missing，链就会被破坏，下一个点将返回空的undefined。您必须从其 get 返回 stub 对象本身。要提供默认值，请覆盖 toPrimitive:

undef = new Proxy({}, {
    get(_, prop) {
        // support some standard object methods, feel free to add more
        
        if (prop === Symbol.toPrimitive)
            return () => 'no such thing'
        if (prop === 'toString')
            return () => 'whatever you want'
        if (prop === 'toJSON')
            return () => 'whatever you want'

        // if asked for a custom prop, return itself
        
        return undef;
    }
});


const safe = obj => new Proxy(obj, {
    get(target, property) {
        let v = target[property];

        if (v === null || typeof v === 'undefined')
            return undef;

        if (typeof v === 'object')
            return safe(v);

        return v;
    }
});

const wrap = safe({foo: {bar: 'hey'}});

console.log(wrap.foo.bar + "!");
console.log(wrap.notExists.xx.yy.zz + "!");

Symbol.toPrimitive is a symbol that specifies a function... called to convert an object to a corresponding primitive value.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/toPrimitive