Symbol.toStringTag

The Symbol.toStringTag static data property represents the well-known symbol 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.

Try it

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

console.log(Object.prototype.toString.call(new ValidatorClass()));
// Expected output: "[object Validator]"

Value

The well-known symbol Symbol.toStringTag.

Property attributes of Symbol.toStringTag
Writableno
Enumerableno
Configurableno

Examples

undefined

Default tags

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

js
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 [Symbol.toStringTag] property. Almost all built-in objects' [Symbol.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 [Symbol.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 [Symbol.toStringTag] property is only used when you explicitly call Object.prototype.toString on it.

js
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:

js
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:

js
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 HTMLButtonElement:

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

Specifications

See also