libuv & The Event Loop — Flow Diagram — Node.js

Event loop — outer phases and inner microtask ring

flowchart TD
    Start(["<b>start</b><br/>call stack empty"]) --> NT
 
    subgraph INNER ["Inner ring — microtasks (drain between every phase)"]
      NT["<b>i · process.nextTick()</b><br/>platinum VIP<br/>drains fully"] --> PR["<b>ii · Promise callbacks</b><br/>gold VIP<br/>drains fully"]
    end
 
    PR --> T["<b>iii · Timer phase</b><br/>setTimeout / setInterval"]
    T --> NT2["drain microtasks"]
    NT2 --> PC["<b>iv · Pending callbacks</b><br/>deferred I/O (TCP errors)"]
    PC --> NT3["drain microtasks"]
    NT3 --> IP["<b>v · Idle / Prepare</b><br/>internal housekeeping"]
    IP --> NT4["drain microtasks"]
    NT4 --> P["<b>vi · Poll phase</b><br/>I/O callbacks: fs, http, crypto<br/>parks here if idle"]
    P --> NT5["drain microtasks"]
    NT5 --> CH["<b>vii · Check phase</b><br/>setImmediate()"]
    CH --> NT6["drain microtasks"]
    NT6 --> CL["<b>viii · Close callbacks</b><br/>socket.on('close')"]
    CL --> NT7["drain microtasks"]
    NT7 --> T
 
    classDef micro fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef timer fill:#fde8e8,stroke:#b91c1c,stroke-width:2px,color:#1a1915;
    classDef poll fill:#eaf2f8,stroke:#3a6ea5,stroke-width:2px,color:#1a1915;
    classDef check fill:#e7efd9,stroke:#587640,stroke-width:2px,color:#1a1915;
    classDef close fill:#ece5f5,stroke:#6b46c1,stroke-width:2px,color:#1a1915;
    classDef drain fill:#f5efe1,stroke:#6a8a4f,stroke-width:1px,color:#1a1915;
    class NT,PR micro
    class T timer
    class P poll
    class CH check
    class CL close
    class PC,IP,NT2,NT3,NT4,NT5,NT6,NT7 drain

process.nextTick()

Platinum VIP. Highest async priority. All queued nextTick callbacks drain first — including any scheduled during the drain.
Promise callbacks

Gold VIP. .then / .catch / .finally and async/await continuations. Drain after nextTick, before any phase.
Timer phase

Executes setTimeout / setInterval callbacks whose delay has elapsed. Delay is a minimum, not guaranteed.
Pending callbacks

Deferred I/O callbacks — e.g. TCP error callbacks that slipped past their own phase.
Idle / Prepare

Internal Node.js housekeeping. Not exposed to user code.
Poll phase

I/O results: fs, http, net, crypto callbacks. The loop parks here when idle, waiting for I/O from the OS.
Check phase

setImmediate() callbacks. Always runs right after Poll — which is why setImmediate beats setTimeout(0) inside I/O.
Close callbacks

socket.on("close") and similar cleanup callbacks. Final stop before the loop cycles back to Timers.

Event loop — outer phases and inner microtask ring

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