Stop the Hand Waving – Scope Chains & Closures

Scope Chains, Closures, Hoisting, and Garbage Collection all have one thing in common: They’re often hand-waved away. How do closures actually work? When does Garbage Collection occur? What really IS the Scope Chain?

In this talk, we will discover it’s not black magic after all; No hand waving is required to explain these language features, in fact you’ve been using them all along without realising.

1. Scope

  • var – lexical scoping
  • let – block scoping
  • const – block scoping

1.5 Hoisting

  • JS does 2 pass parsing.
  • First pass (hoisting) gives scope – variables get ‘hoisted’ up to the top of the function.
  • This is where lots of confusing bugs begin. Google this if you aren’t comfortable with it.

1 Scopes…

  • Scopes can be nested
  • You can access outer scope but not inner scope.
  • You can have multiple inner scopes
  • This does indeed look like a tree of scopes when you look from outer to inner (top down)
  • When you look the other way, inner to outer, you get a chain (bottom up) not a tree

2 Scope chains

  • If you look for a value, JS walks up the scope chain; if nothing has that value you end up in the global scope.

3 Closures

  • Closures occur when an inner scope references a variable in outer scope.
  • This closes over the referenced variable.

4 Garbage Collection

  • GC kicks in when all the code that might need a value has been executed

5 Shadowing

  • This one is a good concept but a lot of people don’t understand it too well
  • When a value is defined more than once in a scope chain, JS reaches the first one and stops; the top-most scope is not closed over.
  • You can avoid a lot of problems here by emulating hoisting and putting all your vars at the top of scope. Use a linter to enforce this!

If you are still not sure:

  • Nodeschool.io
  • npm install -g scope-chains-closures (self paced workshop)