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.

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Description

Many JavaScript types have tags by default:

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

Others have a built-in toStringTag symbol defined:

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]"

Specifications

Browser compatibility

See also

• Object.prototype.toString()