使用Object.assign为House对象设置原型触发TypeError错误求解答

错误原因 #

你定义的House是普通对象，只有构造函数（或class）才有prototype属性，普通对象的prototype是undefined。而Object.assign的第一个参数必须是对象，传入undefined就会抛出Cannot convert undefined or null to object的错误。

正确做法分两种情况： #

1. 给普通对象设置原型

如果想让housePrototype成为House的原型，用Object.setPrototypeOf()方法：

const House = {
    name: 'houseObject',
    readLocation: (location)=>{
        console.log(`house is located at ${location}`)
    }
}

const housePrototype = {
    readLocation: (location) => {
        console.log(`house prototype is located at ${location}`)
    }
}

// 设置House的原型为housePrototype
Object.setPrototypeOf(House, housePrototype)

// 优先调用自身的readLocation
House.readLocation('Malabe'); // 输出：house is located at Malabe
// 删除自身的readLocation后，会调用原型上的方法
delete House.readLocation;
House.readLocation('Malabe'); // 输出：house prototype is located at Malabe
console.log(House);

2. 直接合并属性到House对象

如果只是想把housePrototype的属性合并到House自身（覆盖同名属性），直接把House作为Object.assign的第一个参数：

const House = {
    name: 'houseObject',
    readLocation: (location)=>{
        console.log(`house is located at ${location}`)
    }
}

const housePrototype = {
    readLocation: (location) => {
        console.log(`house prototype is located at ${location}`)
    }
}

// 合并属性到House自身，覆盖原readLocation
Object.assign(House, housePrototype)

House.readLocation('Malabe'); // 输出：house prototype is located at Malabe
console.log(House);

3. 若本意是创建构造函数/类

如果你的目标是定义一个可实例化的类，应该把House定义为构造函数或class，此时才有prototype属性：

// 构造函数写法
function House() {
    this.name = 'houseObject';
}
House.prototype.readLocation = (location)=>{
    console.log(`house is located at ${location}`)
}

const housePrototype = {
    readLocation: (location) => {
        console.log(`house prototype is located at ${location}`)
    }
}

// 合并到构造函数的prototype上
Object.assign(House.prototype, housePrototype)

const myHouse = new House();
myHouse.readLocation('Malabe'); // 输出：house prototype is located at Malabe

内容的提问来源于stack exchange，提问作者Inod Umayanga