Symbol.toStringTag

The Symbol.toStringTag well-known symbol is a string valued property that is used in the creation of the default string description of an object. It is accessed internally by the Object.prototype.toString() method.

Property attributes of Symbol.toStringTag
Writable no
Enumerable no
Configurable no

Examples

Default tags

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

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 classes default to object tag

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

class ValidatorClass {}

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

Custom tag with toStringTag

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 acccess the Symbol.toStringTag property on HTMLButtonElement:

let test = document.createElement('button');
test.toString(); // Returns [object HTMLButtonElement]
test[Symbol.toStringTag];  // Returns HTMLButtonElement

Specifications

Specification
ECMAScript (ECMA-262)
The definition of 'Symbol.toStringTag' in that specification.

Browser compatibility

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See also