The Symbol.toStringTag static data property represents the well-known symbol @@toStringTag. Object.prototype.toString looks up this symbol on the this value for the property containing a string that represents the type of the object.
Value
The well-known symbol @@toStringTag.
Examples
Default tags
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
Built-in toStringTag symbols
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
Custom tag with toStringTag
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]"
toStringTag available on all DOM prototype objects
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
:
const test = document.createElement("button");
test.toString(); // "[object HTMLButtonElement]"
test[Symbol.toStringTag]; // "HTMLButtonElement"