浅谈 javascript 的原型继承
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
// <a href="http://es5.github.com/#x15.2.3.5"target="_blank">http://es5.github.com/#x15.2.3.5</a>
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
// <a href="http://es5.github.com/#x15.2.3.6"target="_blank">http://es5.github.com/#x15.2.3.6</a>
// Patch for WebKit and IE8 standard mode
// Designed by hax <hax.github.com>
// related issue: <a href="https://github.com/kriskowal/es5-shim/issues#issue/5"target="_blank">https://github.com/kriskowal/es5-shim/issues#issue/5</a>
// IE8 Reference:
// <a href="http://msdn.microsoft.com/en-us/library/dd282900.aspx"target="_blank">http://msdn.microsoft.com/en-us/library/dd282900.aspx</a>
// <a href="http://msdn.microsoft.com/en-us/library/dd229916.aspx"target="_blank">http://msdn.microsoft.com/en-us/library/dd229916.aspx</a>
// WebKit Bugs:
// <a href="https://bugs.webkit.org/show_bug.cgi?id=36423"target="_blank">https://bugs.webkit.org/show_bug.cgi?id=36423</a>
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
// <a href="http://es5.github.com/#x15.2.3.7"target="_blank">http://es5.github.com/#x15.2.3.7</a>
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。
[js] view plaincopy
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;
}
}
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