Private class fields use the # prefix for true encapsulation. Here's how to use them effectively.
Basic Syntax#
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Private Methods#
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Private Static Fields#
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Private Accessors#
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Checking for Private Fields#
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Private Fields in Inheritance#
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Factory Pattern with Privacy#
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State Machine#
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Immutable Configuration#
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Observable with Private State#
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Best Practices#
Usage:
✓ Use # for true privacy
✓ Keep sensitive data private
✓ Hide implementation details
✓ Use private methods for internals
Patterns:
✓ Validation in private methods
✓ State management
✓ Factory methods
✓ Encapsulated logic
Benefits:
✓ True encapsulation
✓ No naming conventions needed
✓ Cannot be accessed externally
✓ Safe from prototype pollution
Avoid:
✗ Private fields in object literals
✗ Accessing via ['#name']
✗ Exposing private data in methods
✗ Over-privatizing everything
Conclusion#
Private class fields with # provide true encapsulation in JavaScript. They cannot be accessed or modified from outside the class, making them ideal for sensitive data and internal implementation. Use private methods for validation and internal logic, and remember that each class has its own private namespace even in inheritance.