浅谈javascript的原型继承

javascript的继承在很多框架中都有运用，尤其是原型式继承。首先要理解一个概念，什么是原型式继承？所谓的原型式继承，就是在函数内部先创建一个临时性的构造函数，然后将传入的对象做这个构造函数的原型，最后返回这个临时类型的新实例。

请看源码：

function clone(o) {
    var F = function(){};
    F.prototype = o;
    return new F();
}

首先看ext（4.1的1896行开始）的原型式继承。

var TemplateClass = function(){};
var ExtObject = Ext.Object = {
    chain: function (object) {
        TemplateClass.prototype = object;
        var result = new TemplateClass();
        TemplateClass.prototype = null;
        return result;
    }

}

这里清除了object的prototype。

再看一下jquery是怎么玩的继承。

var jQuery = function( selector, context ) {
        return new jQuery.fn.init( selector, context, rootjQuery );
    };


-----------------------

jQuery.fn = jQuery.prototype = {
    constructor: jQuery,
    init: function( selector, context, rootjQuery ) {
        -----------------------
        }

}

-------------------
jQuery.fn.init.prototype = jQuery.fn;

jquery玩的就比较高，借助jQuery.fn.init来完成，但是思路一样。

 

司徒正美的mass里也有类似的继承，在lang_fix.js里面第17行：

create: function(o){
            if (arguments.length > 1) {
                $.log(" Object.create implementation only accepts the first parameter.")
            }
            function F() {}
            F.prototype = o;
            return new F();
        }

查看了一下es5的官方，找到了他的兼容补丁：

// ES5 15.2.3.5
// http://es5.github.com/#x15.2.3.5
if (!Object.create) {
    Object.create = function create(prototype, properties) {
        var object;
        if (prototype === null) {
            object = { "__proto__": null };
        } else {
            if (typeof prototype != "object") {
                throw new TypeError("typeof prototype["+(typeof prototype)+"] != 'object'");
            }
            var Type = function () {};
            Type.prototype = prototype;
            object = new Type();
            // IE has no built-in implementation of `Object.getPrototypeOf`
            // neither `__proto__`, but this manually setting `__proto__` will
            // guarantee that `Object.getPrototypeOf` will work as expected with
            // objects created using `Object.create`
            object.__proto__ = prototype;
        }
        if (properties !== void 0) {
            Object.defineProperties(object, properties);
        }
        return object;
    };
}

上面的代码考虑的就比较全面，但是需要另外引入Object.defineProperties的补丁才行，源码相对就比较多了。

 

// ES5 15.2.3.6
// http://es5.github.com/#x15.2.3.6

// Patch for WebKit and IE8 standard mode
// Designed by hax <hax.github.com>
// related issue: https://github.com/kriskowal/es5-shim/issues#issue/5
// IE8 Reference:
//     http://msdn.microsoft.com/en-us/library/dd282900.aspx
//     http://msdn.microsoft.com/en-us/library/dd229916.aspx
// WebKit Bugs:
//     https://bugs.webkit.org/show_bug.cgi?id=36423

function doesDefinePropertyWork(object) {
    try {
        Object.defineProperty(object, "sentinel", {});
        return "sentinel" in object;
    } catch (exception) {
        // returns falsy
    }
}

// check whether defineProperty works if it's given. Otherwise,
// shim partially.
if (Object.defineProperty) {
    var definePropertyWorksOnObject = doesDefinePropertyWork({});
    var definePropertyWorksOnDom = typeof document == "undefined" ||
        doesDefinePropertyWork(document.createElement("div"));
    if (!definePropertyWorksOnObject || !definePropertyWorksOnDom) {
        var definePropertyFallback = Object.defineProperty;
    }
}

if (!Object.defineProperty || definePropertyFallback) {
    var ERR_NON_OBJECT_DESCRIPTOR = "Property description must be an object: ";
    var ERR_NON_OBJECT_TARGET = "Object.defineProperty called on non-object: "
    var ERR_ACCESSORS_NOT_SUPPORTED = "getters & setters can not be defined " +
                                      "on this javascript engine";

    Object.defineProperty = function defineProperty(object, property, descriptor) {
        if ((typeof object != "object" && typeof object != "function") || object === null) {
            throw new TypeError(ERR_NON_OBJECT_TARGET + object);
        }
        if ((typeof descriptor != "object" && typeof descriptor != "function") || descriptor === null) {
            throw new TypeError(ERR_NON_OBJECT_DESCRIPTOR + descriptor);
        }
        // make a valiant attempt to use the real defineProperty
        // for I8's DOM elements.
        if (definePropertyFallback) {
            try {
                return definePropertyFallback.call(Object, object, property, descriptor);
            } catch (exception) {
                // try the shim if the real one doesn't work
            }
        }

        // If it's a data property.
        if (owns(descriptor, "value")) {
            // fail silently if "writable", "enumerable", or "configurable"
            // are requested but not supported
            /*
            // alternate approach:
            if ( // can't implement these features; allow false but not true
                !(owns(descriptor, "writable") ? descriptor.writable : true) ||
                !(owns(descriptor, "enumerable") ? descriptor.enumerable : true) ||
                !(owns(descriptor, "configurable") ? descriptor.configurable : true)
            )
                throw new RangeError(
                    "This implementation of Object.defineProperty does not " +
                    "support configurable, enumerable, or writable."
                );
            */

            if (supportsAccessors && (lookupGetter(object, property) ||
                                      lookupSetter(object, property)))
            {
                // As accessors are supported only on engines implementing
                // `__proto__` we can safely override `__proto__` while defining
                // a property to make sure that we don't hit an inherited
                // accessor.
                var prototype = object.__proto__;
                object.__proto__ = prototypeOfObject;
                // Deleting a property anyway since getter / setter may be
                // defined on object itself.
                delete object[property];
                object[property] = descriptor.value;
                // Setting original `__proto__` back now.
                object.__proto__ = prototype;
            } else {
                object[property] = descriptor.value;
            }
        } else {
            if (!supportsAccessors) {
                throw new TypeError(ERR_ACCESSORS_NOT_SUPPORTED);
            }
            // If we got that far then getters and setters can be defined !!
            if (owns(descriptor, "get")) {
                defineGetter(object, property, descriptor.get);
            }
            if (owns(descriptor, "set")) {
                defineSetter(object, property, descriptor.set);
            }
        }
        return object;
    };
}

// ES5 15.2.3.7
// http://es5.github.com/#x15.2.3.7
if (!Object.defineProperties) {
    Object.defineProperties = function defineProperties(object, properties) {
        for (var property in properties) {
            if (owns(properties, property) && property != "__proto__") {
                Object.defineProperty(object, property, properties[property]);
            }
        }
        return object;
    };
}

EcmaScript6的类继承。

class module extends Base {
    constructor() {
    }
}

越玩越像java了，不过es6很多浏览器还不支持。

最后推荐的写法：

if (!Object.create) {
    Object.create = function create(o) {
        var F = function(){};
        F.prototype = o;
        var result = new F();
        F.prototype = null;
        return result;
    }
}