Introduction
Starting with ECMAScript 2015, symbol is a primitive data type, just like number and string.
symbol values are created by calling the Symbol constructor.
1 2 3 | let sym1 = Symbol();let sym2 = Symbol("key"); // optional string key |
Symbols are immutable, and unique.
1 2 3 4 | let sym2 = Symbol("key");let sym3 = Symbol("key");sym2 === sym3; // false, symbols are unique |
Just like strings, symbols can be used as keys for object properties.
1 2 3 4 5 6 7 | let sym = Symbol();let obj = { [sym]: "value"};console.log(obj[sym]); // "value" |
Symbols can also be combined with computed property declarations to declare object properties and class members.
1 2 3 4 5 6 7 8 9 10 | const getClassNameSymbol = Symbol();class C { [getClassNameSymbol](){ return "C"; }}let c = new C();let className = c[getClassNameSymbol](); // "C" |
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.
Here is a list of well-known symbols:
Symbol.hasInstance
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 flatten 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.
Symbol.match
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 String.prototype.replace method.
Symbol.search
A regular expression method that returns the index within a string that matches the regular expression. Called by the String.prototype.search method.
Symbol.species
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 ToPrimitive abstract operation.
Symbol.toStringTag
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.