如前所述,道格拉斯·克罗克福德 (Douglas Crockford) 的电影很好地解释了原因和方法。但将其放入几行 JavaScript 中:
// Declaring our Animal object
var Animal = function () {
this.name = 'unknown';
this.getName = function () {
return this.name;
}
return this;
};
// Declaring our Dog object
var Dog = function () {
// A private variable here
var private = 42;
// overriding the name
this.name = "Bello";
// Implementing ".bark()"
this.bark = function () {
return 'MEOW';
}
return this;
};
// Dog extends animal
Dog.prototype = new Animal();
// -- Done declaring --
// Creating an instance of Dog.
var dog = new Dog();
// Proving our case
console.log(
"Is dog an instance of Dog? ", dog instanceof Dog, "\n",
"Is dog an instance of Animal? ", dog instanceof Animal, "\n",
dog.bark() +"\n", // Should be: "MEOW"
dog.getName() +"\n", // Should be: "Bello"
dog.private +"\n" // Should be: 'undefined'
);
然而,这种方法的问题在于,每次创建对象时都会重新创建该对象。另一种方法是在原型堆栈上声明对象,如下所示:
// Defining test one, prototypal
var testOne = function () {};
testOne.prototype = (function () {
var me = {}, privateVariable = 42;
me.someMethod = function () {
return privateVariable;
};
me.publicVariable = "foo bar";
me.anotherMethod = function () {
return this.publicVariable;
};
return me;
}());
// Defining test two, function
var testTwo = function() {
var me = {}, privateVariable = 42;
me.someMethod = function () {
return privateVariable;
};
me.publicVariable = "foo bar";
me.anotherMethod = function () {
return this.publicVariable;
};
return me;
};
// Proving that both techniques are functionally identical
var resultTestOne = new testOne(),
resultTestTwo = new testTwo();
console.log(
resultTestOne.someMethod(), // Should print 42
resultTestOne.publicVariable // Should print "foo bar"
);
console.log(
resultTestTwo.someMethod(), // Should print 42
resultTestTwo.publicVariable // Should print "foo bar"
);
// Performance benchmark start
var stop, start, loopCount = 1000000;
// Running testOne
start = (new Date()).getTime();
for (var i = loopCount; i>0; i--) {
new testOne();
}
stop = (new Date()).getTime();
console.log('Test one took: '+ Math.round(((stop/1000) - (start/1000))*1000) +' milliseconds');
// Running testTwo
start = (new Date()).getTime();
for (var i = loopCount; i>0; i--) {
new testTwo();
}
stop = (new Date()).getTime();
console.log('Test two took: '+ Math.round(((stop/1000) - (start/1000))*1000) +' milliseconds');
内省时有一个轻微的缺点。转储 testOne 将导致有用信息减少。此外,“testOne”中的私有属性“privateVariable”在所有实例中共享,shesek 的回复中也提到了这一点。