我在
JavaScript中设计一些类层次结构.它到目前为止工作正常,但我看不到如何确定对象是否是父类的“实例”.例：
function BaSEObject(name){
this.name = name;
this.sayWhoAmI = function(){
console.log(this.name + ' is a Derivation1 : ' + (this instanceof Derivation1));
console.log(this.name + ' is a Derivation2 : ' + (this instanceof Derivation2));
console.log(this.name + ' is a BaSEObject : ' + (this instanceof BaSEObject));
};
}
function Derivation1(){
BaSEObject.apply(this,['first derivation']);
}
function Derivation2(){
BaSEObject.apply(this,['second derivation']);
}
var first = new Derivation1();
var second = new Derivation2();
first.sayWhoAmI();记录此：
first derivation is a Derivation1 : true first derivation is a Derivation2 : false first derivation is a BaSEObject : false
而second.sayWhoAmI();记录此：
second derivation is a Derivation1 : false second derivation is a Derivation2 : true second derivation is a BaSEObject : false
我觉得第一个和第二个对象都应该说它们是BaSEObject的实例.
我明白,JavaScript可能不是为了这个,但我想知道是否有办法实现这一点.
解决方法
只调用Base.apply(…)不设置继承.所有这些.apply都是将其设置为第一个参数,没有其他的.调用父构造函数很重要,但这是不够的.
你要做的是正确设置原型链.也就是说,您必须将Derivation1.prototype设置为继承自Base.prototype的内容.
由于构造函数的每个实例都从构造函数的原型继承,您将看到代码
Derivation1.prototype = new Base();
这是一个坏主意,您可以看到为什么：Base期望参数设置实例特定的属性(在这种情况下为name).但是我们不关心这些属性,因为我们稍后在使用Base.apply(this,…)的子构造函数中初始化它们.
所以我们需要的是从Base.prototype继承的对象,幸运的是,ECMASCript 5定义了一个可以为我们做的功能(polyfill)：
Derivation1.prototype = Object.create(Base.prototype);
这将创建一个继承自Base.prototype的新对象.现在,由于您使用新对象替换了原始原型,因此您必须设置构造函数属性,使其正确指向Derivation1：
Derivation1.prototype.constructor = Derivation1;
以下是一个完整的例子.还可以看看this fiddle和this excellent answer by T.J. Crowder,它解释了基本相同的问题,但也许在一个更好的方式.
例：
function BaSEObject(name){
this.name = name;
}
// move properties shared by all instances to the prototype!
BaSEObject.prototype.sayWhoAmI = function() {
console.log(this.name + ' is a Derivation1 : ' + (this instanceof Derivation1));
console.log(this.name + ' is a Derivation2 : ' + (this instanceof Derivation2));
console.log(this.name + ' is a BaSEObject : ' + (this instanceof BaSEObject));
};
function Derivation1(){
BaSEObject.apply(this,['first derivation']);
}
Derivation1.prototype = Object.create(BaSEObject.prototype);
Derivation1.prototype.constructor = Derivation1;
// some useless method of the child "class"
Derivation1.prototype.someOtherMethod = function() {
return 42;
};
var first = new Derivation1();
first.sayWhoAmI();