Module Internals — IIFE, Wrapper & require() — Flow Diagram — Node.js

The 5-step require() pipeline

flowchart LR
    CALL["<b>require('./sum')</b>"]
      --> S1["<b>i · Resolve</b><br/>find absolute path<br/>local / core / npm / json"]
    S1 --> S2["<b>ii · Load</b><br/>read source from disk<br/>into memory"]
    S2 --> S3["<b>iii · Wrap</b><br/>Module Wrapper Function<br/>(exports, require, module,<br/>__filename, __dirname)"]
    S3 --> S4["<b>iv · Evaluate</b><br/>V8 executes the wrapper<br/>module.exports is populated"]
    S4 --> S5["<b>v · Cache</b><br/>stored in memory<br/>keyed by absolute path"]
    S5 --> OUT["<b>vi · return</b><br/>module.exports<br/>handed back to caller"]
    S5 -. "future calls skip steps i-iv" .-> OUT
 
    classDef call fill:#eaf2f8,stroke:#3a6ea5,stroke-width:2px,color:#1a1915;
    classDef early fill:#f5efe1,stroke:#6a8a4f,stroke-width:2px,color:#1a1915;
    classDef wrap fill:#ece5f5,stroke:#6b46c1,stroke-width:2px,color:#1a1915;
    classDef run fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef cache fill:#e7efd9,stroke:#587640,stroke-width:2px,color:#1a1915;
    class CALL,OUT call
    class S1,S2 early
    class S3 wrap
    class S4 run
    class S5 cache

Resolve

Map the string argument to an absolute path. Classifies the target as a local file, a core module, an npm package, or a JSON file.
Load

Read the file bytes from disk into memory. Still just text — not executed yet.
Wrap

Insert the source into `(function(exports, require, module, __filename, __dirname) { ... })`. This is how private scope + the 5 magic parameters appear.
Evaluate

Hand the wrapper to V8. It runs top to bottom; whatever you assign to `module.exports` becomes the return value.
Cache

Store the completed module in an in-memory cache keyed by absolute path. Future require() calls for the same path skip steps i–iv.
Return

The `require()` expression evaluates to whatever `module.exports` is now pointing at — a function, object, class, primitive, anything.

The 5-step require() pipeline

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