Introduction

values are created by calling the Symbol constructor.

Symbols are immutable, and unique.

Just like strings, symbols can be used as keys for object properties.

Symbols can also be combined with computed property declarations to declare object properties and class members.

Well-known Symbols

In addition to user-defined symbols, there are well-known built-in symbols.Built-in symbols are used to represent internal language behaviors.

A method that determines if a constructor object recognizes an object as one of the constructor’s instances. Called by the semantics of the instanceof operator.

Symbol.isConcatSpreadable

A Boolean value indicating that an object should be flattened to its array elements by Array.prototype.concat.

Symbol.iterator

A method that returns the default iterator for an object. Called by the semantics of the for-of statement.

A regular expression method that matches the regular expression against a string. Called by the String.prototype.match method.

Symbol.replace

A regular expression method that replaces matched substrings of a string. Called by the method.

A function valued property that is the constructor function that is used to create derived objects.

Symbol.split

A regular expression method that splits a string at the indices that match the regular expression.Called by the String.prototype.split method.

Symbol.toPrimitive

A method that converts an object to a corresponding primitive value.Called by the abstract operation.

A String value that is used in the creation of the default string description of an object.Called by the built-in method Object.prototype.toString.

Symbol.unscopables

An Object whose own property names are property names that are excluded from the ‘with’ environment bindings of the associated objects.