The well-known symbol @@toStringTag.

Some values do not have Symbol.toStringTag, but have special toString() representations. For a complete list, see Object.prototype.toString().

Object.prototype.toString.call("foo"); // "[object String]"
Object.prototype.toString.call([1, 2]); // "[object Array]"
Object.prototype.toString.call(3); // "[object Number]"
Object.prototype.toString.call(true); // "[object Boolean]"
Object.prototype.toString.call(undefined); // "[object Undefined]"
Object.prototype.toString.call(null); // "[object Null]"
// ... and more

Most built-in objects provide their own @@toStringTag property. Almost all built-in objects' @@toStringTag property is not writable, not enumerable, and configurable; the exception is Iterator, which is writable for compatibility reasons.

For constructor objects like Promise, the property is installed on Constructor.prototype, so that all instances of the constructor inherit @@toStringTag and can be stringified. For non-constructor objects like Math and JSON, the property is installed as a static property, so that the namespace object itself can be stringified. Sometimes, the constructor also provides its own toString method (for example, Intl.Locale), in which case the @@toStringTag property is only used when you explicitly call Object.prototype.toString on it.

Object.prototype.toString.call(new Map()); // "[object Map]"
Object.prototype.toString.call(function* () {}); // "[object GeneratorFunction]"
Object.prototype.toString.call(Promise.resolve()); // "[object Promise]"
// ... and more

When creating your own class, JavaScript defaults to the "Object" tag:

class ValidatorClass {}

Object.prototype.toString.call(new ValidatorClass()); // "[object Object]"

Now, with the help of toStringTag, you are able to set your own custom tag:

class ValidatorClass {
  get [Symbol.toStringTag]() {
    return "Validator";
  }
}

Object.prototype.toString.call(new ValidatorClass()); // "[object Validator]"

Due to a WebIDL spec change in mid-2020, browsers are adding a Symbol.toStringTag property to all DOM prototype objects. For example, to access the Symbol.toStringTag property on HTMLButtonElement:

const test = document.createElement("button");
test.toString(); // "[object HTMLButtonElement]"
test[Symbol.toStringTag]; // "HTMLButtonElement"

• Polyfill of Symbol.toStringTag in core-js
• Object.prototype.toString()