javascript - 为什么这里需要 setter？

Question

HTML 片段

 <div id="container">

    <span class="no"></span>
    <span class="yes"></span>
    <span class="no"></span>
    <span class="no"></span>

 </div>

所以，一个 getter，它返回一个元素 .yes 并提供惰性求值(我认为这就是术语，对吧？

class Something{
    constructor(){
        this.elem = container;
    }

    get childYes(){
        let span = this.elem.querySelector(".yes");
        this.childYes = span;//Cannot set property clientName of #<Docum> which has only a getter
        return span;
    }   
}


var object = new Something();
console.log(object.childYes);

但是如果我添加一个空的 setter 它就可以正常工作:

class Something{
    constructor(){
        this.elem = container;
    }

    get childYes(){
        let span = this.elem.querySelector(".yes");
        this.childYes = span;
        return span;
    }   
    set childYes(a){};
}


var object = new Something();
console.log(object.childYes); // <span class="yes"></span>

我在那里不需要 setter，浏览器到底需要我做什么？

Answer 1

Cannot set property clientName of #<Docum> which has only a getter

通过做 this.childYes = ...在定义为访问器的属性上，您正在尝试使用 setter 。要在实例上定义属性，请使用 Object.defineProperty :

get childYes(){
    let span = this.elem.querySelector(".yes");
    Object.defineProperty(this, "childYes", {
        value: span
    });
    return span;
}   

class Something{
    constructor(){
        this.elem = container;
    }

    get childYes(){
        console.log("Getter ran");
        let span = this.elem.querySelector(".yes");
        Object.defineProperty(this, "childYes", {
            value: span
        });
        return span;
    }   
}

var object = new Something();
console.log(object.childYes); // Runs the getter
console.log(object.childYes); // Uses the data property

<div id="container">
    <span class="no"></span>
    <span class="yes"></span>
    <span class="no"></span>
    <span class="no"></span>
</div>

---

在您提出的评论中:

So, correct me if I'm wrong: After object.childYes is called, the program looks for own .childYes property of object first; fails; goes to the prototype; finds getter; starts to execute the getter and when the line this.childYes = span; appears the program looks for it "here" e.g. in prototype and fails, right?

这不是因为 this.childYes = span; 在哪里线是，但是是的，除此之外它是正确的。当您分配给一个对象属性时，会发生什么取决于该属性是否存在于对象或其原​​型上，如果存在，则该属性是数据属性还是访问器属性:

1. 如果该属性根本不存在(在对象或其任何原型(prototype)上)，则 JavaScript 引擎将其创建为原始对象的数据属性，并赋予其分配的值。
2. 如果它作为数据属性存在(在对象或其任何原型(prototype)上)，引擎会在原始对象上创建或更新它，并为其赋予分配的值。
3. 如果它作为访问器属性存在，则
   1. 如果它有一个setter，它会调用setter
   2. 如果不是，则抛出错误

在您的原始代码中，您在上面的步骤 3.2 处结束，因为该属性作为原型(prototype)上的访问器属性存在。

下面是这些不同场景的示例:

"use strict";

// A function to tell us if an object has a property and, if so
// what kind of property it is
function getPropertyType(obj, propName) {
    const descr = Object.getOwnPropertyDescriptor(obj, propName);
	if (!descr) {
		return "none";
	}
	if (descr.hasOwnProperty("get") || descr.hasOwnProperty("set")) {
		return "accessor";
	}
	if (descr.hasOwnProperty("value") || descr.hasOwnProperty("writable")) {
		return `data (${descr.value})`;
	}
	return "generic"; // Unlikely, but the spec allows for it
}

// An object to use as a prototype
const proto = {
    dataProperty1: "dataProperty1 value",

    _readWrite: "readWriteAccessor default value",
    get readWriteAccessor() {
      return this._readWrite;
    },
    set readWriteAccessor(value) {
      this._readWrite = value;
    },
    
    get readOnlyAccessor() {
      return "readOnlyAccessor value";
    }
};

// Create an object using `proto` as its prototype
const obj = Object.create(proto);

console.log(`obj dataProperty2:       ${getPropertyType(obj, "dataProperty2")}`);
console.log(`proto dataProperty2:     ${getPropertyType(proto, "dataProperty2")}`);

console.log(`--- Before obj.dataProperty1 = "dataProperty1 updated";`);
console.log(`obj dataProperty1:       ${getPropertyType(obj, "dataProperty1")}`);
console.log(`proto dataProperty1:     ${getPropertyType(proto, "dataProperty1")}`);
obj.dataProperty1 = "dataProperty1 updated";
console.log(`--- After obj.dataProperty1 = "dataProperty1 updated";`);
console.log(`obj dataProperty1:       ${getPropertyType(obj, "dataProperty1")}`);
console.log(`proto dataProperty1:     ${getPropertyType(proto, "dataProperty1")}`);

console.log(`--- Before obj.dataProperty2 = "dataProperty2 updated";`);
console.log(`obj dataProperty2:       ${getPropertyType(obj, "dataProperty2")}`);
console.log(`proto dataProperty2:     ${getPropertyType(proto, "dataProperty2")}`);
obj.dataProperty2 = "dataProperty2 updated";
console.log(`--- After obj.dataProperty2 = "dataProperty2 updated";`);
console.log(`obj dataProperty2:       ${getPropertyType(obj, "dataProperty2")}`);
console.log(`proto dataProperty2:     ${getPropertyType(proto, "dataProperty2")}`);

console.log(`--- Before obj.readWriteAccessor = "readWriteAccessor updated";`);
console.log(`obj readWriteAccessor:   ${getPropertyType(obj, "readWriteAccessor")}`);
console.log(`proto readWriteAccessor: ${getPropertyType(proto, "readWriteAccessor")}`);
obj.readWriteAccessor = "readWriteAccessor updated";
console.log(`--- After obj.readWriteAccessor = "readWriteAccessor updated";`);
console.log(`obj readWriteAccessor:   ${getPropertyType(obj, "readWriteAccessor")}`);
console.log(`proto readWriteAccessor: ${getPropertyType(proto, "readWriteAccessor")}`);

console.log(`obj readOnlyAccessor:    ${getPropertyType(obj, "readOnlyAccessor")}`);
console.log(`proto readOnlyAccessor:  ${getPropertyType(proto, "readOnlyAccessor")}`);
console.log(`--- Before obj.readOnlyAccessor = "readOnlyAccessor updated";`);
try {
	obj.readOnlyAccessor = "readOnlyAccessor updated"; // Would fail silently in loose mode, but we're using strict
	console.log(`Worked!`);
} catch (e) {
	console.error(`Assignment failed: ${e.message}`);
}
console.log(`--- After obj.readOnlyAccessor = "readOnlyAccessor updated";`);
console.log(`obj readOnlyAccessor:    ${getPropertyType(obj, "readOnlyAccessor")}`);
console.log(`proto readOnlyAccessor:  ${getPropertyType(proto, "readOnlyAccessor")}`);

.as-console-wrapper {
  max-height: 100% !important;
}