SOLID principles are five essential design rules that help JavaScript developers write cleaner, maintainable code. The guide covers Single Responsibility (one function per purpose), Open/Closed (extend without modifying), Liskov Substitution (subclasses must be replaceable), Interface Segregation (avoid bloated interfaces), and Dependency Inversion (depend on abstractions). Each principle includes practical JavaScript examples showing violations and proper implementations, plus real-world applications and common interview questions.