libuv & Async I/O — Flow Diagram — Node.js

How async code flows through Node.js

flowchart LR
    A["<b>i · Call stack</b><br/>V8 runs sync code<br/>GEC + function frames"]
      -->|"sync"| D["<b>vi · console.log</b><br/>output printed<br/>stack drained"]
    A -->|"async call<br/>(fs / http / timer)"| B["<b>ii · libuv</b><br/>registers callback<br/>uses thread pool + OS"]
    B -->|"OS does the work"| C["<b>iii · OS</b><br/>disk · network · timer<br/>(libuv talks to kernel)"]
    C -->|"result ready"| B
    B -->|"push callback"| Q["<b>iv · Task queues</b><br/>macrotasks · microtasks"]
    Q -->|"event loop checks<br/>'is stack empty?'"| E["<b>v · Event loop</b><br/>moves callback<br/>onto the call stack"]
    E --> A
 
    classDef v8 fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef libuv fill:#e7efd9,stroke:#587640,stroke-width:2px,color:#1a1915;
    classDef os fill:#eaf2f8,stroke:#3a6ea5,stroke-width:2px,color:#1a1915;
    classDef queue fill:#ece5f5,stroke:#6b46c1,stroke-width:2px,color:#1a1915;
    classDef out fill:#f5efe1,stroke:#6a8a4f,stroke-width:2px,color:#1a1915;
    class A,D v8
    class B libuv
    class C os
    class Q queue
    class E out

Call stack (V8)

V8 executes synchronous code one frame at a time — GEC plus any active function contexts.
libuv

C library bundled with Node. Registers async ops with their callbacks and uses the thread pool + OS syscalls to do the work.
Operating system

Disk, network, timers. libuv is the only part of Node that can talk to the kernel.
Task queues

Macrotasks (timers, I/O callbacks) and microtasks (promises, queueMicrotask). Microtasks drain first.
Event loop

Checks whether the call stack is empty; if yes, moves the next ready callback onto it.
Output

Sync output prints immediately; async output prints only after libuv + the event loop hand the callback back to V8.

How async code flows through Node.js

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