Enumeration WellKnownSymbolsConst

The Symbol.asyncIterator symbol is a builtin symbol that is used to access an object's @@asyncIterator method. In order for an object to be async iterable, it must have a Symbol.asyncIterator key.

See: Symbol.asyncIterator

The Symbol.hasInstance well-known symbol is used to determine if a constructor object recognizes an object as its instance. The instanceof operator's behavior can be customized by this symbol.

See: Symbol.hasInstance

The @@isConcatSpreadable symbol (Symbol.isConcatSpreadable) can be defined as an own or inherited property and its value is a boolean. It can control behavior for arrays and array-like objects:

• For array objects, the default behavior is to spread (flatten) elements. Symbol.isConcatSpreadable can avoid flattening in these cases.
• For array-like objects, the default behavior is no spreading or flattening. Symbol.isConcatSpreadable can force flattening in these cases.

See: Symbol.isConcatSpreadable

Whenever an object needs to be iterated (such as at the beginning of a for..of loop), its @@iterator method is called with no arguments, and the returned iterator is used to obtain the values to be iterated.

See: Symbol.iterator

This function is also used to identify if objects have the behavior of regular expressions. For example, the methods String.prototype.startsWith(), String.prototype.endsWith() and String.prototype.includes(), check if their first argument is a regular expression and will throw a TypeError if they are. Now, if the match symbol is set to false (or a Falsy value), it indicates that the object is not intended to be used as a regular expression object.

See: Symbol.match

The Symbol.matchAll well-known symbol returns an iterator, that yields matches of the regular expression against a string. This function is called by the String.prototype.matchAll() method.

See: Symbol.matchAll

The Symbol.replace well-known symbol specifies the method that replaces matched substrings of a string. This function is called by the String.prototype.replace() method.

For more information, RegExp.prototype[@@replace]() and String.prototype.replace().

See: Symbol.replace

The Symbol.search well-known symbol specifies the method that returns the index within a string that matches the regular expression. This function is called by the String.prototype.search() method.

For more information, see RegExp.prototype[@@search]() and String.prototype.search().

See: Symbol.species

The well-known symbol Symbol.species specifies a function-valued property that the constructor function uses to create derived objects. See: Symbol.species

The Symbol.split well-known symbol specifies the method that splits a string at the indices that match a regular expression. This function is called by the String.prototype.split() method.

For more information, see RegExp.prototype[@@split]() and String.prototype.split(). See: Symbol.split

With the help of the Symbol.toPrimitive property (used as a function value), an object can be converted to a primitive value. The function is called with a string argument hint, which specifies the preferred type of the result primitive value. The hint argument can be one of "number", "string", and "default".

See: Symbol.toPrimitive

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.

See: Symbol.toStringTag

The Symbol.unscopables well-known symbol is used to specify an object value of whose own and inherited property names are excluded from the with environment bindings of the associated object.

See: Symbol.unscopables