犀牛书的实例代码：给对象添加freeze， hide， 查询descriptors

Posted 2020-07-17 方方和圆圆

    /*
     * Define a properties() method in Object.prototype that returns an
     * object representing the named properties of the object on which it
     * is invoked (or representing all own properties of the object, if
     * invoked with no arguments).  The returned object defines four useful
     * methods: toString(), descriptors(), hide(), and show().
     */
    (function namespace() {  // Wrap everything in a private function scope

        // This is the function that becomes a method of all object
        function properties() {
            var names;  // An array of property names
            if (arguments.length == 0)  // All own properties of this
                names = Object.getOwnPropertyNames(this);
            else if (arguments.length == 1 && Array.isArray(arguments[0]))
                names = arguments[0];   // Or an array of names
            else                        // Or the names in the argument list
                names = Array.prototype.splice.call(arguments, 0);

            // Return a new Properties object representing the named properties
            return new Properties(this, names);
        }

        // Make it a new nonenumerable property of Object.prototype.
        // This is the only value exported from this private function scope.
        Object.defineProperty(Object.prototype, "properties", {
            value: properties,
            enumerable: false, writable: true, configurable: true
        });

        // This constructor function is invoked by the properties() function above.
        // The Properties class represents a set of properties of an object.
        function Properties(o, names) {
            this.o = o;            // The object that the properties belong to
            this.names = names;    // The names of the properties
        }

        // Make the properties represented by this object nonenumerable
        Properties.prototype.hide = function() {
            var o = this.o, hidden = { enumerable: false };
            this.names.forEach(function(n) {
                if (o.hasOwnProperty(n))
                    Object.defineProperty(o, n, hidden);
            });
            return this;
        };

        // Make these properties read-only and nonconfigurable
        Properties.prototype.freeze = function() {
            var o = this.o, frozen = { writable: false, configurable: false };
            this.names.forEach(function(n) {
                if (o.hasOwnProperty(n))
                    Object.defineProperty(o, n, frozen);
            });
            return this;
        };

        // Return an object that maps names to descriptors for these properties.
        // Use this to copy properties along with their attributes:
        //   Object.defineProperties(dest, src.properties().descriptors());
        Properties.prototype.descriptors = function() {
            var o = this.o, desc = {};
            this.names.forEach(function(n) {
                if (!o.hasOwnProperty(n)) return;
                desc[n] = Object.getOwnPropertyDescriptor(o,n);
            });
            return desc;
        };

        // Return a nicely formatted list of properties, listing the
        // name, value and attributes. Uses the term "permanent" to mean
        // nonconfigurable, "readonly" to mean nonwritable, and "hidden"
        // to mean nonenumerable. Regular enumerable, writable, configurable
        // properties have no attributes listed.
        Properties.prototype.toString = function() {
            var o = this.o; // Used in the nested function below
            var lines = this.names.map(nameToString);
            return "{\n  " + lines.join(",\n  ") + "\n}";

            function nameToString(n) {
                var s = "", desc = Object.getOwnPropertyDescriptor(o, n);
                if (!desc) return "nonexistent " + n + ": undefined";
                if (!desc.configurable) s += "permanent ";
                if ((desc.get && !desc.set) || !desc.writable) s += "readonly ";
                if (!desc.enumerable) s += "hidden ";
                if (desc.get || desc.set) s += "accessor " + n
                else s += n + ": " + ((typeof desc.value==="function")?"function"
                        :desc.value);
                return s;
            }
        };

        // Finally, make the instance methods of the prototype object above
        // nonenumerable, using the methods we‘ve defined here.
        Properties.prototype.properties().hide();
    }()); // Invoke the enclosing function as soon as we‘re done defining it.