JSDN is a formalized way of visually simulating execution of the JavaScript language. It is both a diagram notation and set of simplified execution rules.
JSDN is intended to be a tool for teaching not just the JavaScript language, but general Computer Science concepts as well (heap, stack, closures, garbage collection, etc.).
100% accuracy on internals representation is not a goal of JSDN. Consistency and concepts are favored over intricate details such as language performance optimizations. For example, every time a variable is assigned to a new string, in JSDN we say the string is created, even though the string may already exist depending on the JS runtime / interpretor.