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.

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

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.