Uncaught TypeError: proxy set handler returned false for property '"length"'

Question

我开始测试以下代理模式，并在使用 .splice() 方法时出现标题错误。

class A extends Array {
  constructor(...x) {
    super(...x)
    return new Proxy(this, {
      set(o,p,v) {
        console.log("set: ",o,p,v)
        return o[p] = v
      },
            
      get(o,p) {
        console.log("get: ",o,p)
        return o[p]
      },
    })
  }
}

const a = new A(1,2,3)

a.splice(1,0,"a")

生成以下控制台输出：

get:  Array(3) [ 1, 2, 3 ] splice
get:  Array(3) [ 1, 2, 3 ] length
get:  Array(3) [ 1, 2, 3 ] constructor
set:  Array [] length 0

Uncaught TypeError: proxy set handler returned false for property '"length"'
    InnerModuleEvaluation self-hosted:2411
    InnerModuleEvaluation self-hosted:2411
    evaluation self-hosted:2358

有谁知道我缺少哪些内部细节需要 set 到 return true 在使用 .splice() 的情况下？

Answer 1

这不仅仅是 splice 方法，当尝试设置 non-writable 属性时，所有 strict mode 代码都会发生这种情况。陷阱处理程序的假 return 值表示设置属性失败。

"use strict";
const a = new Proxy([1,2,3], {
  set(o,p,v) {
    console.log("set: ",o,p,v)
    return o[p] = v
  },
});
a.length = 0; // TypeError: 'set' on proxy: trap returned falsish for property 'length'

在你的情况下，问题是你 return 新的属性值，而 a set trap ought to return a boolean. So it "works" when you set the a.length = 5, but not for a.length = 0. Fix this by using the default implementation provided by the Reflect object 对于陷阱，如果你想要的只是记录而不是实际拦截分配，不要忘记 receiver 参数：

const handler = {
  set(o,p,v,r) {
    console.log("set: ",o,p,v)
    return Reflect.set(o,p,v,r);
  },
  get(o,p,r) {
    console.log("get: ",o,p)
    return Reflect.get(o,p,r);
  },
}

class A extends Array {
  constructor(...x) {
    super(...x)
    return new Proxy(this, handler)
  }
}

Uncaught TypeError: proxy set handler returned false for property '"length"'

Uncaught TypeError: proxy set handler returned false for property '"length"'

javascript

oop

es6-proxy