Sync, Async & the magic of setTimeout(0) — Flow Diagram — Node.js

Sync code → call stack empty → libuv callbacks

flowchart TD
    Start["<b>i · Sync code starts</b><br/>console.log(Hello World)<br/>var assignments"]
      --> Block["<b>ii · pbkdf2Sync</b><br/>BLOCKS main thread<br/>(50M iterations)"]
    Block --> AfterBlock["<b>iii · After sync crypto</b><br/>console.log(First Key)"]
    AfterBlock --> Offload["<b>iv · Async offloads</b><br/>setTimeout(0) → timer queue<br/>pbkdf2() → libuv thread pool"]
    Offload --> MoreSync["<b>v · More sync code</b><br/>multiplyFn + console.log"]
    MoreSync --> Empty{"<b>vi · Call stack empty?</b>"}
    Empty -- "yes" --> Loop["<b>vii · Event loop takes over</b>"]
    Loop --> Timer["<b>viii · Timer callback</b><br/>call me right now !!!!"]
    Timer --> Pool["<b>ix · Thread pool callback</b><br/>Second Key is generated"]
 
    classDef sync fill:#f5efe1,stroke:#6a8a4f,stroke-width:2px,color:#1a1915;
    classDef block fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef loop fill:#eaf2f8,stroke:#3a6ea5,stroke-width:2px,color:#1a1915;
    classDef cb fill:#ece5f5,stroke:#6b46c1,stroke-width:2px,color:#1a1915;
    class Start,AfterBlock,MoreSync sync
    class Block,Offload block
    class Empty,Loop loop
    class Timer,Pool cb

Sync starts

V8 pushes GEC, prints "Hello World", assigns `a` and `b` on the call stack.
pbkdf2Sync blocks

Main thread frozen for seconds while V8 computes 50M iterations — event loop cannot advance.
Post-block sync

Once pbkdf2Sync returns, `"First Key is Generated"` prints immediately.
Async offloads

setTimeout(0) → libuv timer queue; pbkdf2() → libuv thread pool. V8 does not wait.
More sync

`multiplyFn()` runs on the call stack, then `"Multiplication result…"` prints.
Stack empty

GEC pops. V8 is idle — the call stack is finally empty.
Event loop

libuv’s event loop starts pushing ready callbacks back onto the call stack.
Timer callback

setTimeout(0) callback fires from the timer queue — `"call me right now !!!!"`.
Thread pool cb

Async pbkdf2 finishes on a worker thread; libuv delivers `"Second Key is generated"`.

Sync code → call stack empty → libuv callbacks

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