JavaScript Core — know what the language is doing
Build a reliable mental model for values, scope, functions, objects, prototypes, coercion, and modules before adding framework complexity.
React and Next.js constantly lean on JavaScript behavior. If this layer is fuzzy, framework rules feel arbitrary.
Read in this order
When this stage feels clear
You can explain why a piece of JavaScript behaves the way it does—not just what it prints.
You can read unfamiliar code without guessing at scope, references, or object lookup.