Symbol - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

In JavaScript, Symbol is a primitive value.

A value having the data type Symbol can be referred to as a "Symbol value". In a JavaScript runtime environment, a symbol value is created by invoking the function Symbol, which dynamically produces an anonymous, unique value. A symbol may be used as an object property.

Symbol can have an optional description, but for debugging purposes only.

A Symbol value represents a unique identifier. For example:

// Here are two symbols with the same description:
let Sym1 = Symbol("Sym")
let Sym2 = Symbol("Sym")

console.log(Sym1 === Sym2) // returns "false"
// Symbols are guaranteed to be unique.
// Even if we create many symbols with the same description,
// they are different values.

Note: If you are familiar with Ruby's (or another language) that also has a feature called "symbols", please don’t be misguided. JavaScript symbols are different.

Symbol type is a new feature in ECMAScript 2015. There is no ECMAScript 5 equivalent for Symbol.

In some programming languages, the symbol data type is referred to as an "atom."

Symbols don't "Auto-Convert" to strings

Most values in JavaScript support implicit conversion to a string. For instance, we can alert almost any value, and it will work. Symbols are special. They don’t auto-convert.

For example:

let Sym = Symbol("Sym")
alert(Sym)  // TypeError: Cannot convert a Symbol value to a string

That’s a "language guard" against messing up, because strings and symbols are fundamentally different, and should not occasionally convert one into another.

If you really want to show a symbol, we need to call .toString() on it.

let Sym = Symbol("Sym")
alert(Sym.toString())  // Symbol(Sym), now it works

Or you can use the symbol.description property to get its description:

let _Sym = Symbol("Sym");

alert(_Sym.description); // Sym

Well-known symbols

The Symbol class has constants for so-called well-known symbols. These symbols let you configure how JS treats an object, by using them as property keys.

Examples of well-known symbols are: Symbol.iterator for array-like objects, or Symbol.search for string objects.

They are listed in the specification in the Well-known symbols table:

• Symbol.hasInstance
• Symbol.isConcatSpreadable
• Symbol.iterator
• Symbol.toPrimitive
• …and so on.

Global symbol registry

There is a global symbol registry holding all available symbols. The methods that access the registry are Symbol.for() and Symbol.keyFor(); these mediate between the global symbol table (or "registry") and the run-time environment. The global symbol registry is mostly built by JavaScript's compiler infrastructure, and the global symbol registry's content is not available to JavaScript's run-time infrastructure, except through these reflective methods.

The method Symbol.for(tokenString) returns a symbol value from the registry, and Symbol.keyFor(symbolValue) returns a token string from the registry; each is the other's inverse, so the following is true:

Symbol.keyFor(Symbol.for("tokenString")) === "tokenString" // true

Learn more

General knowledge

• Symbol (programming) on Wikipedia
• JavaScript data types and data structures
• Symbols in ECMAScript 6
• Symbol in the MDN JS reference
• Object.getOwnPropertySymbols()