Symbol (SA00)

JavaScript Symbols: JavaScript Symbols are primarily intended as unique property keys. They are used as data, not metadata. While well-known symbols like Symbol.iterator allow customizing certain built-in behaviors, they are still used as property keys on objects. They directly participate in the object's behavior during runtime.:

• • Symbol.for() and Symbol.keyFor() (Static Methods):
• ◦ These are static methods because they operate on the global Symbol registry. Symbol.for() registers or retrieves a symbol globally, and Symbol.keyFor() retrieves the key associated with a globally registered symbol. These operations inherently involve a global registry, and individual Symbol instances aren't directly involved. They are accessed via the Symbol object itself.
• • Symbol.toPrimitive (Well-known Symbol, Used as Instance Property):
• ◦ Symbol.toPrimitive is a well-known symbol, not a method or static property. It is meant to be used as a property key on object instances. The value of this property should be a method that JavaScript can call to convert an object to a primitive value (number, string, or boolean). So, Symbol.toPrimitive itself is not a method, but it is used to identify the method on an object that handles primitive conversion. It's a way for you to customize how your objects are coerced into primitive values.
• • Symbol.iterator and other Well-known Symbols (Used as Instance Properties):
• ◦ Symbol.iterator, like Symbol.toPrimitive, is a well-known symbol and serves as an instance property key. Its value on an object is the method that JavaScript uses to iterate over that object (e.g., in a for...of loop). This makes iteration a customizable behavior. Other well-known symbols also provide hooks for customizing various aspects of how objects behave in the language.

Key Differences and Clarifications

• • Static vs. Instance: Static members (Symbol.for, Symbol.keyFor) are associated with the Symbol class itself. Instance members (Symbol.iterator, Symbol.toPrimitive when used as a property) are associated with individual objects.
• • Symbol.toPrimitive is not a method: It's a Symbol instance that, by convention, is used as a key to define a method on your objects that should handle type coercion. It's a way to define how your objects should be converted to primitives, not the conversion method itself.
• • Well-known Symbols are for Customization: Symbol.iterator, Symbol.toPrimitive, and others provide standard "hooks" for you to customize fundamental behaviors (iteration, type coercion) for your own objects.

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It's used when you want an object to be asynchronously iterable (usable with for await...of loops).

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Symbol context:

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ES6 context:

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