如何"覆盖"原型上的已定义(get-)属性？

Question

我有一些代码在原型上定义了一个getter(但没有setter,如果相关的话).返回的值在99.99%的情况下是正确的; 但是,目标是将属性设置为评估特定对象的不同值.

foo = {}
Object.defineProperty(foo, "bar", {
    // only returns odd die sides
    get: function () { return (Math.random() * 6) | 1; }
});

x = Object.create(foo);
x.bar       // => eg. 5
x.bar = 4   // by fair dice roll
x.bar       // nope => eg. 3

如何为现有对象x重写属性,使其可分配(例如,具有默认属性行为)？

附录:虽然可以在x上定义新属性(值或获取/设置),但我正在寻找是否有办法阻止 [原型]中属性的行为并将"条形"变回普通/广告-hoc特定实例的属性.

Answer 1

通过使用Object.defineProperty上x:

var foo = {}
Object.defineProperty(foo, "bar", {
    // only returns odd die sides
    get: function () { return (Math.random() * 6) | 1; }
});

var x = Object.create(foo);
display(x.bar);      // E.g. 5

(function() {
  var bar;
  var proto = Object.getPrototypeOf(x); // Or just use foo

  Object.defineProperty(x, "bar", {
    get: function () { return typeof bar !== "undefined" ? bar : proto.bar; },
    set: function(value) { bar = value; }
  });
})();

display(x.bar);  // Still odd
x.bar = 4;       // By fair dice roll
display(x.bar);  // Shows 4

function display(msg) {
  document.body.insertAdjacentHTML("beforeend", "<p>" + msg + "</p>");
}

我正在寻找是否有办法在[prototype]中停止属性的行为并将"bar"变回普通/ ad-hoc属性.

好的,这有点不同,但仍然使用Object.defineProperty:

var foo = {}
Object.defineProperty(foo, "bar", {
    // only returns odd die sides
    get: function () { return (Math.random() * 6) | 1; }
});

var x = Object.create(foo);
display(x.bar);      // E.g. 5

Object.defineProperty(x, "bar", {
  value: undefined,
  writable: true,
  enumerable: true // Or leave off if you want it non-enumerable
});

display(x.bar);  // undefined
x.bar = 4;       // By fair dice roll
display(x.bar);  // Shows 4

function display(msg) {
  document.body.insertAdjacentHTML("beforeend", "<p>" + msg + "</p>");
}

Answer 2

它并不像 Object.defineProperty 那么简单。
您需要首先获取属性描述符（使用Object.getOwnPropertyDescriptor），然后检查该属性是否可配置。
如果它不可配置，则无法覆盖它。
另外，您需要考虑属性可以具有 getter/setter XOR值。

因此，如果您有 getter/setter，则无法设置desc.value& desc.writable，反之亦然。

let desc = Object.getOwnPropertyDescriptor(self.constructor.prototype, key);

if (desc != null)
{
    // We can only redefine configurable properties !
    if (!desc.configurable)
    {
        console.log("AUTOTRACE-WARNING: Property \"" + key + "\" not configurable ! (" + self.constructor.name + ")");
        continue;
    }

    let g = desc.get != null;
    let s = desc.set != null;

    if (g || s)
    {
        let newDescriptor: PropertyDescriptor = {};
        newDescriptor.enumerable = desc.enumerable;
        newDescriptor.configurable = desc.configurable; 
        // Argh !
        // newDescriptor.value= desc.value; 
        // newDescriptor.writable = desc.writable; 

        if (g)
            newDescriptor.get = getLoggableFunction(desc.get.bind(self), "Property", "get_" + key)

        if (s)
            newDescriptor.set = getLoggableFunction(desc.set.bind(self), "Property", "set_" + key)

        Object.defineProperty(self, key, newDescriptor);

        continue; // if it's a property, it can't be a function 
    } // End if (g || s) 

} // End if (desc != null) 

这是我的做法（注意“TestClass”构造函数中的自动绑定和自动跟踪调用）：

import { autoBind, autoTrace } from "./autobind_autotrace.js";

class TestClass
{

    constructor()
    {
        autoBind(this);
        autoTrace(this);
    }



    get bar(): boolean
    {
        return this._bar;
    }
    set bar(value: boolean)
    {
        this._bar = value;
    }


    public hello()
    {
        console.log("hello", "this", this);
    }


    public world(x, y)
    {
        console.log("world", "this", this);

    }

}

自动绑定/自动跟踪（TypeScript）：

export function autoBind(self: any): any
{
    for (const key of Object.getOwnPropertyNames(self.constructor.prototype))
    {

        if (key !== 'constructor')
        {
            // console.log(key);

            let desc = Object.getOwnPropertyDescriptor(self.constructor.prototype, key);
            

            if (desc != null)
            {
                // We can only redefine configurable properties !
                if (!desc.configurable)
                {
                    console.log("AUTOBIND-WARNING: Property \"" + key + "\" not configurable ! (" + self.constructor.name + ")");
                    continue;
                }

                let g = desc.get != null;
                let s = desc.set != null;

                if (g || s)
                {
                    let newGetter = null;
                    let newSetter = null;

                    if (g)
                        //desc.get = desc.get.bind(self);
                        newGetter = desc.get.bind(self);

                    if (s)
                        // desc.set = desc.set.bind(self);
                        newSetter = desc.set.bind(self);

                    if (newGetter != null && newSetter == null)
                    {
                        Object.defineProperty(self, key, {
                              get: newGetter 
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , value: desc.value
                            // , writable: desc.writable
                        });
                    }
                    else if (newSetter != null && newGetter == null)
                    {
                        Object.defineProperty(self, key, {
                              set: newSetter
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , value: desc.value
                            // , writable: desc.writable
                        });
                    }
                    else // at least one is set, but none of the above cases, so two are set 
                    {
                        Object.defineProperty(self, key, {
                              get: newGetter
                            , set: newSetter
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , value: desc.value
                            // , writable: desc.writable
                        });
                    }

                    // Object.defineProperty(self.constructor.prototype, key, desc);
                    // Object.defineProperty(self.constructor.prototype, key, desc);
                    continue; // if it's a property, it can't be a function 
                } // End if (g || s) 

            } // End if (desc != null) 

            if (typeof (self[key]) === 'function')
            {
                let val = self[key];
                self[key] = val.bind(self);
            }

        } // End if (key !== 'constructor' && typeof val === 'function') 

    } // Next key 

    return self;
} // End Function autoBind



export function autoTrace(self: any): any
{


    function getLoggableFunction_old(func: any, type: any, name: any)
    {
        return function (...args:any[])
        {
            let logText = name + '(';

            for (var i = 0; i < args.length; i++)
            {
                if (i > 0)
                {
                    logText += ', ';
                }
                logText += args[i];
            }
            logText += ');';

            console.log(type + " " + logText);
            return func.apply(self, args);
        };
    }


    function getLoggableFunction(func: any, type: any, name: any)
    {
        return function (...args: any[])
        {
            let logText = name + '(';

            for (var i = 0; i < args.length; i++)
            {
                if (i > 0)
                {
                    logText += ', ';
                }
                logText += args[i];
            }
            logText += ')';

            console.log("Pre " + type + " " + logText + "; ");
            let res = func.apply(self, args);
            console.log("Post " + type + " " + logText + ":", res);
            return res;
        };
    }


    for (const key of Object.getOwnPropertyNames(self.constructor.prototype))
    {

        if (key !== 'constructor')
        {
            // console.log(key);
            let desc = Object.getOwnPropertyDescriptor(self.constructor.prototype, key);

            if (desc != null)
            {
                // We can only redefine configurable properties !
                if (!desc.configurable)
                {
                    console.log("AUTOTRACE-WARNING: Property \"" + key + "\" not configurable ! (" + self.constructor.name + ")");
                    continue;
                }

                let g = desc.get != null;
                let s = desc.set != null;

                if (g || s)
                {
                    let newGetter = null;
                    let newSetter = null;

                    if (g)
                        //desc.get = getLoggableFunction(desc.get.bind(self), "Property", "get_" + key)
                        newGetter = getLoggableFunction(desc.get.bind(self), "Property", "get_" + key)

                    if (s)
                        // desc.set = getLoggableFunction(desc.set.bind(self), "Property", "set_" + key)
                        newSetter = getLoggableFunction(desc.set.bind(self), "Property", "set_" + key)

                    if (newGetter != null && newSetter == null)
                    {
                        Object.defineProperty(self, key, {
                              get: newGetter
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , value: desc.value
                            // , writable: desc.writable
                        });
                    }
                    else if (newSetter != null && newGetter == null)
                    {
                        Object.defineProperty(self, key, {
                              set: newSetter
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , value: desc.value
                            // , writable: desc.writable
                        });
                    }
                    else // at least one is set, but none of the above cases, so two are set 
                    {
                        Object.defineProperty(self, key, {
                              get: newGetter
                            , set: newSetter
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , value: desc.value
                            // , writable: desc.writable
                        });
                    }

                    continue; // if it's a property, it can't be a function 
                } // End if (g || s) 

            } // End if (desc != null) 

            // if it's not a property, it can only be a function or not a function 
            if (typeof (self[key]) === 'function')
            {
                let val = self[key];
                self[key] = getLoggableFunction(val.bind(self), "Function", key);
            } // End if (typeof (self[key]) === 'function') 

        } // End if (key !== 'constructor' && typeof val === 'function') 

    } // Next key 

    return self;
} // End Function autoTrace

这会转换为纯 JavaScript：

"use strict";

function autoBind(self)
{
    for (var _i = 0, _a = Object.getOwnPropertyNames(self.constructor.prototype); _i < _a.length; _i++)
    {
        var key = _a[_i];
        if (key !== 'constructor')
        {
            var desc = Object.getOwnPropertyDescriptor(self.constructor.prototype, key);
            
            if (desc != null)
            {
                if (!desc.configurable)
                {
                    console.log("AUTOBIND-WARNING: Property \"" + key + "\" not configurable ! (" + self.constructor.name + ")");
                    continue;
                }
                var g = desc.get != null;
                var s = desc.set != null;
                if (g || s)
                {
                    var newGetter = null;
                    var newSetter = null;
                    if (g)
                        newGetter = desc.get.bind(self);
                    if (s)
                        newSetter = desc.set.bind(self);
                    if (newGetter != null && newSetter == null)
                    {
                        Object.defineProperty(self, key, {
                              get: newGetter 
                            , enumerable: desc.enumerable 
                            , configurable: desc.configurable 
                            // , writable: desc.writable 
                            // , value: desc.value 
                        });
                    }
                    else if (newSetter != null && newGetter == null)
                    {
                        Object.defineProperty(self, key, {
                              set: newSetter 
                            , enumerable: desc.enumerable 
                            , configurable: desc.configurable 
                            // , writable: desc.writable 
                            // , value: desc.value 
                        });
                    }
                    else
                    {
                        Object.defineProperty(self, key, {
                              get: newGetter 
                            , set: newSetter
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , writable: desc.writable
                            // , value: desc.value
                        });
                    }
                    continue;
                }
            }
            if (typeof (self[key]) === 'function')
            {
                var val = self[key];
                self[key] = val.bind(self);
            }
        }
    }
    return self;
}

function autoTrace(self)
{
    function getLoggableFunction_old(func, type, name)
    {
        return function ()
        {
            var args = [];
            for (var _i = 0; _i < arguments.length; _i++)
            {
                args[_i] = arguments[_i];
            }
            var logText = name + '(';
            for (var i = 0; i < args.length; i++)
            {
                if (i > 0)
                {
                    logText += ', ';
                }
                logText += args[i];
            }
            logText += ');';
            console.log(type + " " + logText);
            return func.apply(self, args);
        };
    }
    function getLoggableFunction(func, type, name)
    {
        return function ()
        {
            var args = [];
            for (var _i = 0; _i < arguments.length; _i++)
            {
                args[_i] = arguments[_i];
            }
            var logText = name + '(';
            for (var i = 0; i < args.length; i++)
            {
                if (i > 0)
                {
                    logText += ', ';
                }
                logText += args[i];
            }
            logText += ')';
            console.log("Pre " + type + " " + logText + "; ");
            var res = func.apply(self, args);
            console.log("Post " + type + " " + logText + ":", res);
            return res;
        };
    }
    for (var _i = 0, _a = Object.getOwnPropertyNames(self.constructor.prototype); _i < _a.length; _i++)
    {
        var key = _a[_i];
        if (key !== 'constructor')
        {
            var desc = Object.getOwnPropertyDescriptor(self.constructor.prototype, key);
            if (desc != null)
            {
                if (!desc.configurable)
                {
                    console.log("AUTOTRACE-WARNING: Property \"" + key + "\" not configurable ! (" + self.constructor.name + ")");
                    continue;
                }
                var g = desc.get != null;
                var s = desc.set != null;
                if (g || s)
                {
                    var newGetter = null;
                    var newSetter = null;
                    if (g)
                        newGetter = getLoggableFunction(desc.get.bind(self), "Property", "get_" + key);
                    if (s)
                        newSetter = getLoggableFunction(desc.set.bind(self), "Property", "set_" + key);
                    if (newGetter != null && newSetter == null)
                    {
                        Object.defineProperty(self, key, {
                              get: newGetter 
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , writable: desc.writable
                            // , value: desc.value
                        });
                    }
                    else if (newSetter != null && newGetter == null)
                    {
                        Object.defineProperty(self, key, {
                              set: newSetter 
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , writable: desc.writable
                            // , value: desc.value
                        });
                    }
                    else
                    {
                        Object.defineProperty(self, key, {
                              get: newGetter 
                            , set: newSetter 
                            , enumerable: desc.enumerable
                            , configurable: desc.configurable
                            // , writable: desc.writable
                            // , value: desc.value
                        });
                    }
                    continue;
                }
            }
            if (typeof (self[key]) === 'function')
            {
                var val = self[key];
                self[key] = getLoggableFunction(val.bind(self), "Function", key);
            }
        }
    }
    return self;
}

// exports and module are globals of "require" - delete if usage without require 
exports = exports || {}; // prevent error if no module 
exports.autoBind = autoBind;
exports.autoTrace = autoTrace;