SymboltoStringTag

Symbol.toStringTag is a well-known symbol introduced in ECMAScript 2015 that customizes the result of Object.prototype.toString. It provides a way to influence the tag used in the default string representation of objects.

How it works: Object.prototype.toString returns a string like [object Type], where Type reflects the object's internal tag. If the object has a property keyed by Symbol.toStringTag whose value is a string, that string is used as the Type. For example, Object.prototype.toString.call(obj) yields [object Custom] when obj[Symbol.toStringTag] = 'Custom'. If the property is absent or not a string, the default internal tag is used.

Defining and usage: You can set Symbol.toStringTag on an object or its prototype to affect all instances. Example: const o = { [Symbol.toStringTag]: 'MyType' }; Object.prototype.toString.call(o); // "[object MyType]". For classes, assigning on the prototype also affects instances. Class C {}, C.prototype[Symbol.toStringTag] = 'C'; Object.prototype.toString.call(new C()); // "[object C]".

Notes: The Symbol.toStringTag property is not enumerable and does not affect JSON serialization or other stringification methods; it specifically alters toString output. It is a symbol-keyed property, retrievable via Object.getOwnPropertySymbols, but not usually visible in ordinary property enumerations.

See also: Object.prototype.toString, Symbol, well-known symbols.