Polyfills, this/call/apply/bind, Currying, Debounce, Promises & Coding Challenges

The closure family tree — one pattern, many problems

flowchart TD
    C["<b>i · Closure primitive</b><br/>outer fn returns inner fn<br/>inner closes over private state"]
      --> RL["<b>ii · Rate limiters</b><br/>private state = timer / flag"]
    C --> IG["<b>iii · Invocation gates</b><br/>private state = cache / called"]
    C --> FT["<b>iv · Functional transformers</b><br/>private state = accumulated args"]
    RL --> DB["<b>v · Debounce</b><br/>clearTimeout + setTimeout<br/>last call wins"]
    RL --> TH["<b>vi · Throttle</b><br/>boolean flag + setTimeout<br/>first call per interval"]
    IG --> ON["<b>vii · Once</b><br/>boolean called + cached result"]
    IG --> MEM["<b>viii · Memoize</b><br/>Map cache keyed by JSON.stringify(args)"]
    FT --> CU["<b>ix · Curry</b><br/>collect args until fn.length reached"]
    FT --> PC["<b>x · Pipe / Compose</b><br/>reduce / reduceRight through fns"]
 
    classDef root   fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef branch fill:#f5efe1,stroke:#6a8a4f,stroke-width:2px,color:#1a1915;
    classDef leaf   fill:#e7efd9,stroke:#587640,stroke-width:2px,color:#1a1915;
    class C root
    class RL,IG,FT branch
    class DB,TH,ON,MEM,CU,PC leaf

• i

  Closure primitive

  Every pattern here returns a new function that closes over some private state — timers, flags, caches, or accumulated args.

• ii

  Rate limiters

  Control how often a function can fire. Private state is a timer handle or a boolean flag.

• iii

  Invocation gates

  Control whether the function runs at all. Private state is a called flag or a results cache.

• iv

  Functional transformers

  Reshape how a function is called. Private state is either accumulated args (curry) or an ordered list of functions (pipe/compose).

• v

  Debounce

  clearTimeout + setTimeout — runs fn delay ms after the LAST call. Good for search-as-you-type.

• vi

  Throttle

  Boolean `inThrottle` flag — runs fn at most once per interval. Good for scroll/resize.

• vii

  Once

  Runs exactly once; subsequent calls return the cached result. Useful for init paths and "pay once" flows.

• viii

  Memoize

  Cache keyed by JSON.stringify(args). Skips recomputation for identical inputs.

• ix

  Curry

  Collect args across calls; invoke the original once args.length ≥ fn.length. Enables partial application.

• x

  Pipe / Compose

  Thread a value through a list of functions. pipe goes left→right (reduce); compose goes right→left (reduceRight).