Closures — The Complete Guide

Closure formation — the variable that outlived its context

flowchart TD
    S["<b>i · Parent function runs</b><br/>new EC pushed onto Call Stack"]
      --> M["<b>ii · Parent's memory created</b><br/>var a = 7; function inner(){…}"]
    M --> R["<b>iii · Return inner</b><br/>function + reference to lexical env"]
    R --> D["<b>iv · Parent EC destroyed</b><br/>popped off the Call Stack"]
    D --> K["<b>v · Closure kept alive</b><br/>a = 7 survives via reference"]
    K --> C["<b>vi · Inner called later</b><br/>new EC for inner is pushed"]
    C --> L["<b>vii · Scope chain lookup</b><br/>inner's local → closed-over a"]
    L --> O["<b>viii · Returns 7</b><br/>variable survived the parent's death"]
 
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    classDef warn fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef boundary fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef node fill:#f5efe1,stroke:#6a8a4f,stroke-width:2px,color:#1a1915;
    class S,M,R,D,C,L node
    class K warn
    class O ok

• i

  Parent runs

  Call stack pushes the parent function's Execution Context — a fresh lexical environment is created.

• ii

  Memory created

  Parent's variables (like `var a = 7`) and inner function declarations get their memory slots.

• iii

  Return inner

  The inner function is returned along with its reference to the parent's lexical environment — not just its code.

• iv

  Parent EC destroyed

  Parent returns. Its Execution Context is popped off the Call Stack.

• v

  Variable preserved

  Because the returned inner function still references them, the parent's variables escape garbage collection. That persistence is the closure.

• vi

  Inner called later

  When the caller eventually invokes the returned function, a new EC is pushed for `inner` — its lexical parent reference still points at the preserved environment.

• vii

  Scope chain lookup

  Reading `a` inside inner misses local, walks the outer reference, and finds the closed-over `a = 7`.

• viii

  Value returned

  Inner prints / returns 7. The parent is long gone, but the variable survived inside the closure.