[ ] and wrapped keysSymbol.iterator and not Symbol.ITERATOR (etc.)?
Symbol
Symbols are a new primitive type in ECMAScript 6. They are created via a factory function:
const mySymbol = Symbol('mySymbol');
Every time you call the factory function, a new and unique symbol is created. The optional parameter is a descriptive string that is shown when printing the symbol (it has no other purpose):
Symbols are mainly used as unique property keys – a symbol never clashes with any other property key (symbol or string). For example, you can make an object iterable (usable via the for-of loop and other language mechanisms), by using the symbol stored in Symbol.iterator as the key of a method (more information on iterables is given in the chapter on iteration):
const iterableObject = {
[Symbol.iterator]() { // (A)
···
}
}
for (const x of iterableObject) {
console.log(x);
}
// Output:
// hello
// world
In line A, a symbol is used as the key of the method. This unique marker makes the object iterable and enables us to use the for-of loop.
In ECMAScript 5, you may have used strings to represent concepts such as colors. In ES6, you can use symbols and be sure that they are always unique:
const COLOR_RED = Symbol('Red');
const COLOR_ORANGE = Symbol('Orange');
const COLOR_YELLOW = Symbol('Yellow');
const COLOR_GREEN = Symbol('Green');
const COLOR_BLUE = Symbol('Blue');
const COLOR_VIOLET = Symbol('Violet');
function getComplement(color) {
switch (color) {
case COLOR_RED:
return COLOR_GREEN;
case COLOR_ORANGE:
return COLOR_BLUE;
case COLOR_YELLOW:
return COLOR_VIOLET;
case COLOR_GREEN:
return COLOR_RED;
case COLOR_BLUE:
return COLOR_ORANGE;
case COLOR_VIOLET:
return COLOR_YELLOW;
default:
throw new Exception('Unknown color: '+color);
}
}
Every time you call Symbol('Red'), a new symbol is created. Therefore, COLOR_RED can never be mistaken for another value. That would be different if it were the string 'Red'.
Coercing (implicitly converting) symbols to strings throws exceptions:
const sym = Symbol('desc');
const str1 = '' + sym; // TypeError
const str2 = `${sym}`; // TypeError
The only solution is to convert explicitly:
const str2 = String(sym); // 'Symbol(desc)'
const str3 = sym.toString(); // 'Symbol(desc)'
Forbidding coercion prevents some errors, but also makes working with symbols more complicated.
The following operations are aware of symbols as property keys:
Reflect.ownKeys()[]
Object.assign()The following operations ignore symbols as property keys:
Object.keys()Object.getOwnPropertyNames()for-in loopECMAScript 6 introduces a new primitive type: symbols. They are tokens that serve as unique IDs. You create symbols via the factory function Symbol() (which is loosely similar to String returning strings if called as a function):
const symbol1 = Symbol();
Symbol() has an optional string-valued parameter that lets you give the newly created Symbol a description. That description is used when the symbol is converted to a string (via toString() or String()):
Every symbol returned by Symbol() is unique, every symbol has its own identity:
You can see that symbols are primitive if you apply the typeof operator to one of them – it will return a new symbol-specific result:
Symbols can be used as property keys:
const MY_KEY = Symbol();
const obj = {};
obj[MY_KEY] = 123;
console.log(obj[MY_KEY]); // 123
Classes and object literals have a feature called computed property keys: You can specify the key of a property via an expression, by putting it in square brackets. In the following object literal, we use a computed property key to make the value of MY_KEY the key of a property.
const MY_KEY = Symbol();
const obj = {
[MY_KEY]: 123
};
A method definition can also have a computed key:
const FOO = Symbol();
const obj = {
[FOO]() {
return 'bar';
}
};
console.log(obj[FOO]()); // bar
Given that there is now a new kind of value that can become the key of a property, the following terminology is used for ECMAScript 6:
Let’s examine the APIs for enumerating own property keys by first creating an object.
const obj = {
[Symbol('my_key')]: 1,
enum: 2,
nonEnum: 3
};
Object.defineProperty(obj,
'nonEnum', { enumerable: false });
Object.getOwnPropertyNames() ignores symbol-valued property keys:
Object.getOwnPropertySymbols() ignores string-valued property keys:
Reflect.ownKeys() considers all kinds of keys:
Object.keys() only considers enumerable property keys that are strings:
The name Object.keys clashes with the new terminology (only string keys are listed). Object.names or Object.getEnumerableOwnPropertyNames would be a better choice now.
In ECMAScript 5, one often represents concepts (think enum constants) via strings. For example:
var COLOR_RED = 'Red';
var COLOR_ORANGE = 'Orange';
var COLOR_YELLOW = 'Yellow';
var COLOR_GREEN = 'Green';
var COLOR_BLUE = 'Blue';
var COLOR_VIOLET = 'Violet';
However, strings are not as unique as we’d like them to be. To see why, let’s look at the following function.
function getComplement(color) {
switch (color) {
case COLOR_RED:
return COLOR_GREEN;
case COLOR_ORANGE:
return COLOR_BLUE;
case COLOR_YELLOW:
return COLOR_VIOLET;
case COLOR_GREEN:
return COLOR_RED;
case COLOR_BLUE:
return COLOR_ORANGE;
case COLOR_VIOLET:
return COLOR_YELLOW;
default:
throw new Exception('Unknown color: '+color);
}
}
It is noteworthy that you can use arbitrary expressions as switch cases, you are not limited in any way. For example:
function isThree(x) {
switch (x) {
case 1 + 1 + 1:
return true;
default:
return false;
}
}
We use the flexibility that switch offers us and refer to the colors via our constants (COLOR_RED etc.) instead of hard-coding them ('Red' etc.).
Interestingly, even though we do so, there can still be mix-ups. For example, someone may define a constant for a mood:
var MOOD_BLUE = 'Blue';
Now the value of COLOR_BLUE is not unique anymore and MOOD_BLUE can be mistaken for it. If you use it as a parameter for getComplement(), it returns 'Orange' where it should throw an exception.
Let’s use symbols to fix this example. Now we can also use the ES6 feature const, which lets us declare actual constants (you can’t change what value is bound to a constant, but the value itself may be mutable).
const COLOR_RED = Symbol('Red');
const COLOR_ORANGE = Symbol('Orange');
const COLOR_YELLOW = Symbol('Yellow');
const COLOR_GREEN = Symbol('Green');
const COLOR_BLUE = Symbol('Blue');
const COLOR_VIOLET = Symbol('Violet');
Each value returned by Symbol is unique, which is why no other value can be mistaken for BLUE now. Intriguingly, the code of getComplement() doesn’t change at all if we use symbols instead of strings, which shows how similar they are.
Being able to create properties whose keys never clash with other keys is useful in two situations:
Whenever there are inheritance hierarchies in JavaScript (e.g. created via classes, mixins or a purely prototypal approach), you have two kinds of properties:
For usability’s sake, public properties usually have string keys. But for private properties with string keys, accidental name clashes can become a problem. Therefore, symbols are a good choice. For example, in the following code, symbols are used for the private properties _counter and _action.
const _counter = Symbol('counter');
const _action = Symbol('action');
class Countdown {
constructor(counter, action) {
this[_counter] = counter;
this[_action] = action;
}
dec() {
let counter = this[_counter];
if (counter < 1) return;
counter--;
this[_counter] = counter;
if (counter === 0) {
this[_action]();
}
}
}
Note that symbols only protect you from name clashes, not from unauthorized access, because you can find out all own property keys – including symbols – of an object via Reflect.ownKeys(). If you want protection there, as well, you can use one of the approaches listed in Sect. “Private data for classes”.
Symbols having unique identities makes them ideal as keys of public properties that exist on a different level than “normal” property keys, because meta-level keys and normal keys must not clash. One example of meta-level properties are methods that objects can implement to customize how they are treated by a library. Using symbol keys protects the library from mistaking normal methods as customization methods.
ES6 Iterability is one such customization. An object is iterable if it has a method whose key is the symbol (stored in) Symbol.iterator. In the following code, obj is iterable.
const obj = {
data: [ 'hello', 'world' ],
[Symbol.iterator]() {
···
}
};
The iterability of obj enables you to use the for-of loop and similar JavaScript features:
for (const x of obj) {
console.log(x);
}
// Output:
// hello
// world
In case you think that name clashes don’t matter, here are three examples of where name clashes caused problems in the evolution of the JavaScript standard library:
Array.prototype.values() was created, it broke existing code where with was used with an Array and shadowed a variable values in an outer scope (bug report 1, bug report 2). Therefore, a mechanism was introduced to hide properties from with (Symbol.unscopables).String.prototype.contains clashed with a method added by MooTools and had to be renamed to String.prototype.includes (bug report).Array.prototype.contains also clashed with a method added by MooTools and had to be renamed to Array.prototype.includes (bug report).In contrast, adding iterability to an object via the property key Symbol.iterator can’t cause problems, because that key doesn’t clash with anything.
The following table shows what happens if you explicitly or implicitly convert symbols to other primitive types:
| Conversion to | Explicit conversion | Coercion (implicit conversion) |
|---|---|---|
| boolean |
Boolean(sym) → OK |
!sym → OK |
| number |
Number(sym) → TypeError
|
sym*2 → TypeError
|
| string |
String(sym) → OK |
''+sym → TypeError
|
sym.toString() → OK |
`${sym}` → TypeError
|
Coercion to string being forbidden can easily trip you up:
const sym = Symbol();
console.log('A symbol: '+sym); // TypeError
console.log(`A symbol: ${sym}`); // TypeError
To fix these problems, you need an explicit conversion to string:
console.log('A symbol: '+String(sym)); // OK
console.log(`A symbol: ${String(sym)}`); // OK
Coercion (implicit conversion) is often forbidden for symbols. This section explains why.
Coercion to boolean is always allowed, mainly to enable truthiness checks in if statements and other locations:
if (value) { ··· }
param = param || 0;
Symbols are special property keys, which is why you want to avoid accidentally converting them to strings, which are a different kind of property keys. This could happen if you use the addition operator to compute the name of a property:
myObject['__' + value]
That’s why a TypeError is thrown if value is a symbol.
You also don’t want to accidentally turn symbols into Array indices. The following is code where that could happen if value is a symbol:
myArray[1 + value]
That’s why the addition operator throws an error in this case.
To explicitly convert a symbol to boolean, you call Boolean(), which returns true for symbols:
Boolean() computes its result via the internal operation ToBoolean(), which returns true for symbols and other truthy values.
Coercion also uses ToBoolean():
To explicitly convert a symbol to number, you call Number():
Number() computes its result via the internal operation ToNumber(), which throws a TypeError for symbols.
Coercion also uses ToNumber():
To explicitly convert a symbol to string, you call String():
If the parameter of String() is a symbol then it handles the conversion to string itself and returns the string Symbol() wrapped around the description that was provided when creating the symbol. If no description was given, the empty string is used:
The toString() method returns the same string as String(), but neither of these two operations calls the other one, they both call the same internal operation SymbolDescriptiveString().
Coercion is handled via the internal operation ToString(), which throws a TypeError for symbols. One method that coerces its parameter to string is Number.parseInt():
+) The addition operator works as follows:
ToString()), concatenate them and return the result.Coercion to either string or number throws an exception, which means that you can’t (directly) use the addition operator for symbols:
While all other primitive values have literals, you need to create symbols by function-calling Symbol. Thus, there is a risk of accidentally invoking Symbol as a constructor. That produces instances of Symbol, which are not very useful. Therefore, an exception is thrown when you try to do that:
There is still a way to create wrapper objects, instances of Symbol: Object, called as a function, converts all values to objects, including symbols.
[ ] and wrapped keys The square bracket operator [ ] normally coerces its operand to string. There are now two exceptions: symbol wrapper objects are unwrapped and symbols are used as they are. Let’s use the following object to examine this phenomenon.
const sym = Symbol('yes');
const obj = {
[sym]: 'a',
str: 'b',
};
The square bracket operator unwraps wrapped symbols:
Like any other value not related to symbols, a wrapped string is converted to a string by the square bracket operator:
The operator for getting and setting properties uses the internal operation ToPropertyKey(), which works as follows:
ToPrimitive() with the preferred type String:
[@@toPrimitive](), that method is used to convert it to a primitive value. Symbols have such a method, which returns the wrapped symbol.toString() – if it returns a primitive value. Otherwise, valueOf() is used – if it returns a primitive value. Otherwise, a TypeError is thrown. The preferred type String determines that toString() is called first, valueOf() second.ToString().A code realm (short: realm) is a context in which pieces of code exist. It includes global variables, loaded modules and more. Even though code exists “inside” exactly one realm, it may have access to code in other realms. For example, each frame in a browser has its own realm. And execution can jump from one frame to another, as the following HTML demonstrates.
<head>
<script>
function test(arr) {
var iframe = frames[0];
// This code and the iframe’s code exist in
// different realms. Therefore, global variables
// such as Array are different:
console.log(Array === iframe.Array); // false
console.log(arr instanceof Array); // false
console.log(arr instanceof iframe.Array); // true
// But: symbols are the same
console.log(Symbol.iterator ===
iframe.Symbol.iterator); // true
}
</script>
</head>
<body>
<iframe srcdoc="<script>window.parent.test([])</script>">
</iframe>
</body>
The problem is that each realm has its own global variables where each variable Array points to a different object, even though they are all essentially the same object. Similarly, libraries and user code are loaded once per realm and each realm has a different version of the same object.
Objects are compared by identity, but booleans, numbers and strings are compared by value. Therefore, no matter in which realm a number 123 originated, it is indistinguishable from all other 123s. That is similar to the number literal 123 always producing the same value.
Symbols have individual identities and thus don’t travel across realms as smoothly as other primitive values. That is a problem for symbols such as Symbol.iterator that should work across realms: If an object is iterable in one realm, it should be iterable in all realms. All built-in symbols are managed by the JavaScript engine, which makes sure that, e.g., Symbol.iterator is the same value in each realm. If a library wants to provide cross-realm symbols, it has to rely on extra support, which comes in the form of the global symbol registry: This registry is global to all realms and maps strings to symbols. For each symbol, the library needs to come up with a string that is as unique as possible. To create the symbol, it doesn’t use Symbol(), it asks the registry for the symbol that the string is mapped to. If the registry already has an entry for the string, the associated symbol is returned. Otherwise, entry and symbol are created first.
You ask the registry for a symbol via Symbol.for() and retrieve the string associated with a symbol (its key) via Symbol.keyFor():
Cross-realm symbols, such as Symbol.iterator, that are provided by the JavaScript engine, are not in the registry:
The original plan was for symbols to support private properties (there would have been public and private symbols). But that feature was dropped, because using “get” and “set” (two meta-object protocol operations) for managing private data does not interact well with proxies:
These two goals are at odds. The chapter on classes explains your options for managing private data. Symbols is one of these options, but you don’t get the same amount of safety that you’d get from private symbols, because it’s possible to determine the symbols used as an object’s property keys, via Object.getOwnPropertySymbols() and Reflect.ownKeys().
In some ways, symbols are like primitive values, in other ways, they are like objects:
What are symbols then – primitive values or objects? In the end, they were turned into primitives, for two reasons.
First, symbols are more like strings than like objects: They are a fundamental value of the language, they are immutable and they can be used as property keys. Symbols having unique identities doesn’t necessarily contradict them being like strings: UUID algorithms produce strings that are quasi-unique.
Second, symbols are most often used as property keys, so it makes sense to optimize the JavaScript specification and implementations for that use case. Then symbols don’t need many abilities of objects:
instanceof, Object.keys(), etc.Symbols not having these abilities makes life easier for the specification and the implementations. The V8 team has also said that when it comes to property keys, it is easier to make a primitive type a special case than certain objects.
In contrast to strings, symbols are unique and prevent name clashes. That is nice to have for tokens such as colors, but it is essential for supporting meta-level methods such as the one whose key is Symbol.iterator. Python uses the special name __iter__ to avoid clashes. You can reserve double underscore names for programming language mechanisms, but what is a library to do? With symbols, we have an extensibility mechanism that works for everyone. As you can see later, in the section on public symbols, JavaScript itself already makes ample use of this mechanism.
There is one hypothetical alternative to symbols when it comes to clash-free property keys: using a naming convention. For example, strings with URLs (e.g. 'http://example.com/iterator'). But that would introduce a second category of property keys (versus “normal” property names that are usually valid identifiers and don’t contain colons, slashes, dots, etc.), which is basically what symbols are, anyway. Then we may just as well introduce a new kind of value.
No, they are not.
Ruby’s symbols are basically literals for creating values. Mentioning the same symbol twice produces the same value twice:
:foo == :foo
The JavaScript function Symbol() is a factory for symbols – each value it returns is unique:
Symbol('foo') !== Symbol('foo')
Symbol.iterator and not Symbol.ITERATOR (etc.)? Well-known symbols are stored in properties whose names start with lowercase characters and are camel-cased. In a way, these properties are constants and it is customary for constants to have all-caps names (Math.PI etc.). But the reasoning for their spelling is different: Well-known symbols are used instead of normal property keys, which is why their “names” follow the rules for property keys, not the rules for constants.
This section gives an overview of the ECMAScript 6 API for symbols.
Symbol
Symbol(description?) : symbol
Creates a new symbol. The optional parameter description allows you to give the symbol a description. The only way to access the description is to convert the symbol to a string (via toString() or String()). The result of such a conversion is 'Symbol('+description+')':
Symbol is can’t be used as a constructor – an exception is thrown if you invoke it via new.
The only useful method that symbols have is toString() (via Symbol.prototype.toString()).
| Conversion to | Explicit conversion | Coercion (implicit conversion) |
|---|---|---|
| boolean |
Boolean(sym) → OK |
!sym → OK |
| number |
Number(sym) → TypeError
|
sym*2 → TypeError
|
| string |
String(sym) → OK |
''+sym → TypeError
|
sym.toString() → OK |
`${sym}` → TypeError
|
|
| object |
Object(sym) → OK |
Object.keys(sym) → OK |
The global object Symbol has several properties that serve as constants for so-called well-known symbols. These symbols let you configure how ES6 treats an object, by using them as property keys. This is a list of all well-known symbols:
Symbol.hasInstance (method)C customize the behavior of x instanceof C.Symbol.toPrimitive (method)Symbol.toStringTag (string)Object.prototype.toString() to compute the default string description of an object obj:'[object ' + obj[Symbol.toStringTag] + ']'
Symbol.unscopables (Object)with statement.Symbol.iterator (method)for-of loop and the spread operator (...)). The method returns an iterator. Details: chapter “Iterables and iterators”.String.prototype.match(x, ···) is forwarded to x[Symbol.match](···).String.prototype.replace(x, ···) is forwarded to x[Symbol.replace](···).String.prototype.search(x, ···) is forwarded to x[Symbol.search](···).String.prototype.split(x, ···) is forwarded to x[Symbol.split](···).The details are explained in Sect. “String methods that delegate regular expression work to their parameters” in the chapter on strings.
Symbol.species (method)Array.prototype.map()) create objects that are similar to this. The details are explained in the chapter on classes.Symbol.isConcatSpreadable (boolean)Array.prototype.concat() adds the indexed elements of an object to its result (“spreading”) or the object as a single element (details are explained in the chapter on Arrays).If you want a symbol to be the same in all realms, you need to use the global symbol registry, via the following two methods:
Symbol.for(str) : symbolstr in the registry. If str isn’t in the registry yet, a new symbol is created and filed in the registry under the key str.Symbol.keyFor(sym) : stringsym in the registry. If sym isn’t in the registry, this method returns undefined. This method can be used to serialize symbols (e.g. to JSON).