我正在尝试在我的 JS 应用程序中实现一种继承。 这是我所拥有的:
function FooObject(param1, param2) {
this._param1= param1;
this._param2= param2;
}
Object.defineProperties(FooObject.prototype, {
param1:
{
get: function () { return this._param1; },
set: function (val) { this._param1= val; }
}
});
上面是一个“基类”,好吗?
然后我定义了另一个对象,它通过合并扩展了 FooObject 并添加了更多属性:
function FooObjectA(param1, param2, param3, param4) {
FooObject.call(this, param1, someVal);
this._param2= param2;
this._param3= param3;
this._param4= param4;
}
FooObjectA.prototype = Object.assign(Object.create(FooObject.prototype),
{
constructor: FooObjectA,
param2:
{
get: function () { return this._param2; },
set: function (val) { this._param2= val; }
},
param3:
{
get: function () { return this._param3; },
set: function (val) { this._param3 = val; }
}
});
现在,如果我尝试获取 param3,例如:
var fooInstance = new FooObjectA();
var p = fooInstance .param3;
然后 p
不保存 _param3 的值,而是引用 getter 方法。
然后我尝试了这个方法:
Object.defineProperties(FooObjectA.prototype, {
{
param2:
{
get: function () { return this._param2; },
set: function (val) { this._param2= val; }
},
param3:
{
get: function () { return this._param3; },
set: function (val) { this._param3 = val; }
}
});
现在它从 getter 返回值 ok。我是 JavaScript 的新手,想了解为什么当我将 getters/setters 放入 .assign..create... 时它不起作用,因为从逻辑上讲我看不出为什么这些 getters 没有与原型(prototype)合并FooObject.
最佳答案
Object.assign
读取属性的值(调用 getter)并将该值作为简单数据属性存储在目标对象上。
如果您想复制 getter/setter 定义,您需要实现自己的函数来检查属性描述符 (Object.getOwnPropertyDescriptor(obj, "propName")
) 并处理创建目标上的 getter/setter 属性。
then p doesn't hold the value of _param3 but reference to the getter method.
那是一个单独的问题。您正在将一个对象传递给 Object.assign
,并希望它按照 Object.defineProperties
的方式进行解释:
FooObjectA.prototype = smartAssign(Object.create(FooObject.prototype), {
constructor: FooObjectA,
param2: {
get: function() {
return this._param2;
},
set: function(val) {
this._param2 = val;
}
},
param3: {
get: function() {
return this._param3;
},
set: function(val) {
this._param3 = val;
}
}
});
如果你想使用那种格式,你需要调用Object.defineProperties
来解释它:
FooObjectA.prototype = smartAssign(Object.defineProperties(Object.create(FooObject.prototype), {
// --------------------------------^
constructor: FooObjectA,
param2: {
get: function() {
return this._param2;
},
set: function(val) {
this._param2 = val;
}
},
param3: {
get: function() {
return this._param3;
},
set: function(val) {
this._param3 = val;
}
}
})); // <=== Added )
这是执行此操作的函数(使用 ES5 语法)的快速草图;您很可能需要测试并强化它:
var hasOwn = function(obj, prop) { // Paranoia, in case an object overrides it
return Object.prototype.hasOwnProperty.call(obj, prop);
};
function smartAssign(target) {
var n, source, key;
for (n = 1; n < arguments.length; ++n) {
source = arguments[n];
for (key in source) {
if (hasOwn(source, key)) {
Object.defineProperty(target, key, Object.getOwnPropertyDescriptor(source, key));
}
}
}
return target;
}
使用它并修复了另一个问题的实例:
var hasOwn = function(obj, prop) {
return Object.prototype.hasOwnProperty.call(obj, prop);
};
function smartAssign(target) {
var n, source, key;
for (n = 1; n < arguments.length; ++n) {
source = arguments[n];
for (key in source) {
if (hasOwn(source, key)) {
Object.defineProperty(target, key, Object.getOwnPropertyDescriptor(source, key));
}
}
}
return target;
}
function FooObject(param1, param2) {
this._param1 = param1;
this._param2 = param2;
}
Object.defineProperties(FooObject.prototype, {
param1: {
get: function() {
return this._param1;
},
set: function(val) {
this._param1 = val;
}
}
});
function FooObjectA(param1, param2, param3, param4) {
FooObject.call(this, param1, 42);
this._param2 = param2;
this._param3 = param3;
this._param4 = param4;
}
FooObjectA.prototype = smartAssign(Object.defineProperties(Object.create(FooObject.prototype), {
constructor: FooObjectA,
param2: {
get: function() {
return this._param2;
},
set: function(val) {
this._param2 = val;
}
},
param3: {
get: function() {
return this._param3;
},
set: function(val) {
this._param3 = val;
}
}
}));
var fooInstance = new FooObjectA("p1", "p2", "p3", "p4");
console.log(fooInstance.param1);
console.log(fooInstance.param3);
关于javascript - 为什么 .assign 对象合并中的 setter/getter 定义不起作用?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/50270256/