Node.js中‘type tags’（类型标签）的含义是什么？

Great question! Let's unpack what type tags refer to when Node.js's assert.deepStrictEqual() checks that "object type tags should be the same."

You're right that typeof date and typeof fakeDate both return object—but that's a very high-level, generic type check. The "type tags" here are referring to the internal classification of the object revealed by Object.prototype.toString.call(), not the limited output of the typeof operator.

Let's test this with your example code to see the difference:

const date = new Date();
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);

console.log(Object.prototype.toString.call(date)); // Logs: [object Date]
console.log(Object.prototype.toString.call(fakeDate)); // Logs: [object Object]

Even though fakeDate inherits from Date.prototype, it was originally created as a plain object. This means its internal type tag remains Object, while the real Date instance has a specific type tag of Date.

assert.deepStrictEqual() enforces this check to ensure you're comparing objects of the same fundamental type, not just objects that share a prototype chain. This prevents misleading matches where a plain object mimics another type's prototype but isn't actually a genuine instance of that type.

To sum it up:

• typeof gives you a broad, vague category (like object)
• Type tags are the precise internal label that identifies exactly what kind of object you're working with (e.g., Date, Array, Map, RegExp)

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