{"componentChunkName":"component---src-templates-handbook-tsx","path":"/docs/handbook/symbols.html","result":{"data":{"markdownRemark":{"id":"f0ee06cc-16db-513e-ab34-a22cd6df01b6","excerpt":"Introduction Starting with ECMAScript 2015, symbol is a primitive data type, just like number and string. symbol values are created by calling the Symbol…","html":"

Introduction

\n

Starting with ECMAScript 2015, symbol is a primitive data type, just like number and string.

\n

symbol values are created by calling the Symbol constructor.

\n
ts
let sym1 = Symbol();\n\nlet sym2 = Symbol(\"key\"); // optional string key
\n

Symbols are immutable, and unique.

\n
ts
let sym2 = Symbol(\"key\");\nlet sym3 = Symbol(\"key\");\n\nsym2 === sym3; // false, symbols are unique
\n

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

\n
ts
const sym = Symbol();\n\nlet obj = {\n    [sym]: \"value\"\n};\n\nconsole.log(obj[sym]); // \"value\"
\n

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

\n
ts
const getClassNameSymbol = Symbol();\n\nclass C {\n    [getClassNameSymbol](){\n       return \"C\";\n    }\n}\n\nlet c = new C();\nlet className = c[getClassNameSymbol](); // \"C\"
\n

Well-known Symbols

\n

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

\n

Here is a list of well-known symbols:

\n

Symbol.hasInstance

\n

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.

\n

Symbol.isConcatSpreadable

\n

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

\n

Symbol.iterator

\n

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

\n

Symbol.match

\n

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

\n

Symbol.replace

\n

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

\n

Symbol.search

\n

A regular expression method that returns the index within a string that matches the regular expression. Called by the String.prototype.search method.

\n

Symbol.species

\n

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

\n

Symbol.split

\n

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

\n

Symbol.toPrimitive

\n

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

\n

Symbol.toStringTag

\n

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

\n

Symbol.unscopables

\n

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

","headings":[{"value":"Introduction","depth":1},{"value":"Well-known Symbols","depth":1},{"value":"Symbol.hasInstance","depth":2},{"value":"Symbol.isConcatSpreadable","depth":2},{"value":"Symbol.iterator","depth":2},{"value":"Symbol.match","depth":2},{"value":"Symbol.replace","depth":2},{"value":"Symbol.search","depth":2},{"value":"Symbol.species","depth":2},{"value":"Symbol.split","depth":2},{"value":"Symbol.toPrimitive","depth":2},{"value":"Symbol.toStringTag","depth":2},{"value":"Symbol.unscopables","depth":2}],"frontmatter":{"permalink":"/docs/handbook/symbols.html","title":"Symbols"}}},"pageContext":{"slug":"/docs/handbook/symbols.html","isOldHandbook":true}}}