The well-known symbolSymbol.toStringTag.

Some values do not haveSymbol.toStringTag, but have specialtoString() representations. For a complete list, seeObject.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[Symbol.toStringTag] property. Almost all built-in objects'[Symbol.toStringTag] property is not writable, not enumerable, and configurable; the exception isIterator, which is writable for compatibility reasons.

For constructor objects likePromise, the property is installed onConstructor.prototype, so that all instances of the constructor inherit[Symbol.toStringTag] and can be stringified. For non-constructor objects likeMath andJSON, the property is installed as a static property, so that the namespace object itself can be stringified. Sometimes, the constructor also provides its owntoString method (for example,Intl.Locale), in which case the[Symbol.toStringTag] property is only used when you explicitly callObject.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 oftoStringTag, 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 aWebIDL spec change in mid-2020, browsers are adding aSymbol.toStringTag property to all DOM prototype objects. For example, to access theSymbol.toStringTag property onHTMLButtonElement:

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

• Polyfill ofSymbol.toStringTag incore-js
• Object.prototype.toString()