Modules, Exports & Require — Flow Diagram — Node.js

module.exports → require — the only legal bridge

flowchart LR
    subgraph SUM ["sum.js (private module)"]
        A["<b>i · body</b><br/>let x = ...<br/>function calculateSum()"]
          --> B["<b>ii · module.exports</b><br/>= { x, calculateSum }"]
    end
 
    subgraph APP ["app.js (private module)"]
        D["<b>iv · require('./sum')</b><br/>receives module.exports"]
          --> E["<b>v · destructure</b><br/>const { x, calculateSum } = …"]
        E --> F["<b>vi · use it</b><br/>calculateSum(5, 10)"]
    end
 
    B -->|"iii · bridge<br/>CJS require()"| D
 
    G["<b>vii · barrel</b><br/>folder/index.js<br/>re-exports siblings"] -.->|alt path| D
 
    classDef priv fill:#f5efe1,stroke:#6a8a4f,stroke-width:2px,color:#1a1915;
    classDef pub fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef use fill:#eaf2f8,stroke:#3a6ea5,stroke-width:2px,color:#1a1915;
    classDef barrel fill:#ece5f5,stroke:#6b46c1,stroke-width:2px,color:#1a1915;
    class A,F priv
    class B,D pub
    class E use
    class G barrel

Module body

Variables and functions live in the file's private scope. Nothing leaks by default.
module.exports

The public door. Whatever you assign here becomes the module's return value.
CJS bridge

require("./sum") runs the file once, then returns its module.exports value.
require() in consumer

Other file receives whatever was exported — a function, an object, a class, anything.
Destructure

The professional pattern: const { x, calculateSum } = require("./sum"). Clean, explicit, scannable.
Use the import

Call the function, read the variable — just as if it were defined locally.
Barrel (index.js)

Put index.js inside a folder to group exports; require("./folder") auto-loads it.

module.exports → require — the only legal bridge

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