javascript - 如何用不是原始类型的正确类型覆盖 valueOf()？

Question

假设我需要通过 Array 类型覆盖内部 valueOf()...

作为explained here valueOf() 方法返回指定对象的原始值...所以，我想我们可以将原始值设置为任何数据类型...可以吗？还是有限制？

这里是相同的链接示例，仅更改返回和测试:

function MyNumberType(n) {
  this.number = n;
}
MyNumberType.prototype.valueOf = function() {
  return [123,this.number];
};

const object1 = new MyNumberType(4);    
console.log(object1 +  [1,1]); // expected output: 123,41,1

Answer 1

Many operators and functions in JavaScript expect their arguments to have certain types. If they don’t, they are coerced (converted) to those types. Coercing an object to a primitive type is a two-step process: First, the object is converted to a primitive. Then, if necessary, the primitive is converted to the correct type. Two methods are used to convert an object to a primitive: valueOf() toString() ¹

让我们看看将两个对象相乘时会发生什么:

function Foo() {
}

// valueOf returns an object which is not a primitive
Foo.prototype.valueOf = () => {
  console.log('converting to primitive');
  return {};
};

// valueOf returns an object which is not a primitive
Foo.prototype.toString = () => {
  console.log('converting to string');
  return 42;
};

const foo1 = new Foo();
const foo2 = new Foo();

console.log(foo1 * foo2);

如您所见，首先调用 valueOf() 将对象转换为其原始表示形式。然而，对象不是原始对象，因此接下来调用 toString()。

您的代码的问题在于，正如@Thomas 所指出的，valueOf() 不返回原始值。因此调用 toString() 返回标准的 [object Object] 除非你覆盖它:

function MyNumberType(n) {
  this.number = n;
}

MyNumberType.prototype.valueOf = function() {
  // force `toString()` to be called
  console.log('converting to primitive');
  return {};
};

MyNumberType.prototype.toString = function() {
  console.log('converting to string');
  return `123,${this.number}`;
};

const object1 = new MyNumberType(4);    
console.log(object1 +  [1,1]); // expected output: 123,41,1

引用资料

1. > http://2ality.com/2012/11/coercing-objects.html