Documentation
Mastering JavaScript
A focused path through the runtime: how code is parsed, scheduled, and executed — with interactive diagrams and high-signal explanations.
Topics
- Execution ContextVariable environment and thread of execution — how the engine runs your code one line at a time.
- Call stack & phasesCreation vs execution phases, invocations, and how the engine pushes and pops execution contexts on the call stack.
- HoistingCreation-phase bindings, var vs function declarations, undefined vs not defined, and function expressions.
- undefined vs not definedundefined is a real placeholder value the engine assigns before execution — not defined is a ReferenceError for names that were never declared.
- Variable environment & invocationSame name in global vs function code: per-call memory, shadowing, call stack, and console order 10 · 100 · 1.
- The Shortest JS ProgramEven an empty file triggers the engine to create a global object (window), set up this, and build a Global Execution Context.
- Event LoopCall stack, tasks, microtasks, and how async work fits in.