theitroad>Tutorial JavaScript>JavaScript(JS) Symbols
tutorial

JavaScript(JS) Symbols

What is Symbol

In JavaScript, a symbol is a primitive data type that represents a unique identifier. Symbols were introduced in ECMAScript 2015 (ES6) and are used to create property keys that are guaranteed to be unique.

Here's an example of creating a symbol:

‮fer‬er to:theitroad.com
let mySymbol = Symbol();

You can also provide a description for the symbol, which can be useful for debugging purposes:

let mySymbol = Symbol("My Symbol");

Symbols are guaranteed to be unique, even if they have the same description:

let symbol1 = Symbol("My Symbol");
let symbol2 = Symbol("My Symbol");

console.log(symbol1 === symbol2); // Output: false

Symbols can be used as property keys in objects:

let mySymbol = Symbol("My Symbol");
let myObject = {};

myObject[mySymbol] = "Hello, world!";

console.log(myObject[mySymbol]); // Output: "Hello, world!"

Using symbols as property keys ensures that they will not conflict with other property keys, even if they have the same name. This can be useful in cases where you need to define properties with unique names, such as when working with libraries or frameworks.

Creating Symbol

In JavaScript, you can create a new symbol using the Symbol() function. The Symbol() function returns a unique symbol every time it's called, even if you pass the same parameter to it. Here's an example:

let mySymbol = Symbol();
console.log(typeof mySymbol); // Output: "symbol"

You can also pass a description to the Symbol() function, which can be useful for debugging purposes:

let mySymbol = Symbol("My Symbol");
console.log(mySymbol.toString()); // Output: "Symbol(My Symbol)"

Symbols with the same description are not equal to each other, even though they have the same name:

let symbol1 = Symbol("My Symbol");
let symbol2 = Symbol("My Symbol");

console.log(symbol1 === symbol2); // Output: false

Symbols can also be used as property keys in objects:

let mySymbol = Symbol("My Symbol");
let myObject = {};

myObject[mySymbol] = "Hello, world!";
console.log(myObject[mySymbol]); // Output: "Hello, world!"

Using symbols as property keys ensures that they will not conflict with other property keys, even if they have the same name. This can be useful in cases where you need to define properties with unique names, such as when working with libraries or frameworks.

Access Symbol Description

In JavaScript, you can access the description of a symbol using the description property. This property returns the description that was provided when the symbol was created using the Symbol() function.

Here's an example:

let mySymbol = Symbol("My Symbol");
console.log(mySymbol.description); // Output: "My Symbol"

If you don't provide a description when creating a symbol, the description property returns an empty string:

let mySymbol = Symbol();
console.log(mySymbol.description); // Output: ""

It's important to note that the description property is only available in ES6 (ECMAScript 2015) and later versions of JavaScript. If you're working with an older version of JavaScript, you can't access the description of a symbol. In that case, you can use the toString() method to get a string representation of the symbol, which includes the description:

let mySymbol = Symbol("My Symbol");
console.log(mySymbol.toString()); // Output: "Symbol(My Symbol)"

However, this method returns a string, not a symbol, so you can't use it as a property key in objects or compare it with other symbols using the === operator.

Add Symbol as an Object Key

In JavaScript, you can use symbols as object keys using the bracket notation. Here's an example:

let mySymbol = Symbol("My Symbol");
let myObject = {};

myObject[mySymbol] = "Hello, world!";
console.log(myObject[mySymbol]); // Output: "Hello, world!"

In this example, we create a new symbol mySymbol and an empty object myObject. Then, we use the bracket notation to set the value of the property with the key mySymbol to "Hello, world!". Finally, we use the bracket notation again to access the value of the property with the key mySymbol.

Using symbols as object keys ensures that they will not conflict with other property keys, even if they have the same name. This can be useful in cases where you need to define properties with unique names, such as when working with libraries or frameworks.

It's important to note that symbols are not enumerable by default, which means they won't show up when you use the for...in loop or the Object.keys() method. To access the symbols in an object, you can use the Object.getOwnPropertySymbols() method:

let mySymbol = Symbol("My Symbol");
let myObject = {};

myObject[mySymbol] = "Hello, world!";
let symbols = Object.getOwnPropertySymbols(myObject);
console.log(symbols); // Output: [Symbol(My Symbol)]
console.log(myObject[symbols[0]]); // Output: "Hello, world!"

In this example, we use the Object.getOwnPropertySymbols() method to get an array of symbols in myObject. Then, we access the value of the property with the first symbol in the array.

Symbols are not included in for...in Loop

In JavaScript, symbols are not included in the for...in loop. The for...in loop is used to iterate over the enumerable properties of an object. Enumerable properties are those that can be iterated over in a for...in loop or accessed using the Object.keys() method.

However, symbols are not enumerable by default, which means they won't show up when you use the for...in loop or the Object.keys() method. To access the symbols in an object, you can use the Object.getOwnPropertySymbols() method:

let mySymbol = Symbol("My Symbol");
let myObject = {};

myObject[mySymbol] = "Hello, world!";
let symbols = Object.getOwnPropertySymbols(myObject);
console.log(symbols); // Output: [Symbol(My Symbol)]
console.log(myObject[symbols[0]]); // Output: "Hello, world!"

In this example, we use the Object.getOwnPropertySymbols() method to get an array of symbols in myObject. Then, we access the value of the property with the first symbol in the array.

It's important to note that the for...in loop is not recommended for iterating over arrays, because it can also iterate over inherited properties and properties with non-numeric keys. Instead, you can use a for loop or the Array.forEach() method to iterate over arrays.

Symbol Methods

In JavaScript, symbols have several methods that you can use to manipulate and access information about them. Here are some of the most commonly used symbol methods:

  1. Symbol.for(key): This method creates a new symbol or returns an existing symbol with the given key. The symbol is stored in a global symbol registry, which allows symbols to be shared across different parts of your code.

  2. Symbol.keyFor(symbol): This method returns the key for a symbol that has been stored in the global symbol registry using Symbol.for(). If the symbol was not created using Symbol.for(), this method returns undefined.

  3. Symbol.prototype.toString(): This method returns a string representation of the symbol. This string includes the symbol's unique identifier, which is generated when the symbol is created.

  4. Symbol.prototype.valueOf(): This method returns the primitive value of the symbol. This value is the same as the symbol's unique identifier, which is generated when the symbol is created.

Here's an example that demonstrates how to use these methods:

let mySymbol = Symbol.for("My Symbol");

console.log(mySymbol); // Output: Symbol(My Symbol)
console.log(mySymbol.valueOf()); // Output: Symbol(My Symbol)
console.log(mySymbol.toString()); // Output: "Symbol(My Symbol)"
console.log(Symbol.keyFor(mySymbol)); // Output: "My Symbol"

In this example, we create a new symbol with the key "My Symbol" using Symbol.for(). Then, we use the valueOf() and toString() methods to get the primitive value and string representation of the symbol. Finally, we use the Symbol.keyFor() method to get the key for the symbol from the global symbol registry.

Symbol Properties

In JavaScript, symbols have several properties that you can use to access information about them. Here are some of the most commonly used symbol properties:

  1. Symbol.iterator: This is a built-in symbol that represents the default iterator for an object. You can use this symbol to define custom iterators for your own objects.

  2. Symbol.species: This is a built-in symbol that is used to define the constructor function for a derived object. For example, if you have a subclass of Array, you can use this symbol to specify the constructor function that should be used to create new instances of the subclass.

  3. Symbol.match: This is a built-in symbol that is used to define the regular expression method that should be called when a string is matched against a regular expression.

  4. Symbol.replace: This is a built-in symbol that is used to define the regular expression method that should be called when a string is replaced using a regular expression.

  5. Symbol.search: This is a built-in symbol that is used to define the regular expression method that should be called when a string is searched using a regular expression.

  6. Symbol.split: This is a built-in symbol that is used to define the regular expression method that should be called when a string is split using a regular expression.

Here's an example that demonstrates how to use some of these properties:

let mySymbol = Symbol.iterator;
let myArray = [1, 2, 3];

let iterator = myArray[mySymbol]();

console.log(iterator.next()); // Output: { value: 1, done: false }
console.log(iterator.next()); // Output: { value: 2, done: false }
console.log(iterator.next()); // Output: { value: 3, done: false }
console.log(iterator.next()); // Output: { value: undefined, done: true }

In this example, we use the Symbol.iterator property to get the default iterator for the myArray object. We then use the iterator to iterate over the values in the array. The next() method of the iterator returns an object with two properties: value, which is the next value in the iteration, and done, which is a boolean that indicates whether the iteration is complete.