Eye Training · Functions, this and Closures

this is decided by how a function is called, not where it's written. A closure keeps whatever it can see, for as long as it lives.

Part 2 · Surface 22. Bug counts are hidden and some snippets are clean. See Start Here for how to use these drills.

J · The basics

M · Three years in, these should jump out

S · Where seniors stand out

What your eye checks

Whenever a function is passed around, stored, or defined inside another, ask:

1. How will it be called? Passed as a callback (addEventListener, map, setTimeout) or destructured, a method loses its object.
2. Arrow or function? Arrows take this from where they're written: right for callbacks inside methods, wrong as object methods.
3. Does the arrow body return? { } needs return; an object literal needs ({ }).
4. Is it defined before it's used? const and let have a temporal dead zone; function declarations are hoisted.
5. Is a stateful function created once or on every call? Debounced, throttled and memoized functions must be created once.
6. What does the closure capture, and how long does it live? A long-lived callback keeps every variable it refers to.
7. What happens on the second call? once, memoize and init helpers must return the stored result.
8. Does recursion have a way out? Base cases, depth limits, cycles.

Tells

Go deeper

• Callbacks Done Wrong: this, callbacks and the habits that cause these bugs
• Polyfills: writing bind, once, debounce and memoize correctly
• Previous surface: Loops and iteration · Next: Time and dates