JavaScript values, functions and scope

In this roadmap JavaScript runs in the browser and, for most stacks, on the server too. Gaps here show up everywhere. Learn let and const, and skip var.

Learn the difference between primitives (numbers, strings, true and false) and objects. Objects are shared by reference: if a function changes one, the caller sees the change. Use === rather than ==, and learn which values count as truthy and falsy.

Get fluent with arrow functions, destructuring, spread and the array methods map, filter, find and reduce, because React code is full of them. Understand scope and closures: a function remembers the variables around the place where it was created.