Symbols are unique, immutable primitive values used for property keys and metaprogramming. Here's how to use them effectively.
Basic Usage
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Hidden Properties
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Symbol.for() - Global Registry
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Well-Known Symbols
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Symbol.toStringTag
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Symbol.toPrimitive
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Symbol.hasInstance
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Symbol.species
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Symbol.isConcatSpreadable
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Symbol.match/replace/search/split
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Private-like Properties
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Plugin/Extension Keys
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Type Checking with Symbols
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Best Practices
Usage:
✓ Unique property keys
✓ Hidden metadata
✓ Well-known symbol customization
✓ Avoiding name collisions
Global Registry:
✓ Use Symbol.for() for shared symbols
✓ Use descriptive key names
✓ Namespace keys (e.g., 'myapp.feature')
✓ Check Symbol.keyFor() when needed
Well-Known Symbols:
✓ Symbol.iterator for iterables
✓ Symbol.toStringTag for debugging
✓ Symbol.toPrimitive for coercion
✓ Symbol.hasInstance for instanceof
Avoid:
✗ Symbols for true privacy (use #private)
✗ Overusing symbols
✗ Forgetting symbols are not enumerable
✗ Converting symbols to strings accidentally
Conclusion
Symbols provide unique identifiers for object properties and enable metaprogramming through well-known symbols. Use them for hidden properties, avoiding naming collisions in libraries, and customizing object behavior like iteration and type coercion. The global registry (Symbol.for) enables cross-realm symbol sharing. While not truly private, symbols are hidden from most iteration methods, making them useful for metadata and internal properties.