$function

The $function operator lets you define and execute custom logic using JavaScript-like syntax within a pipeline.

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📌 Syntax

{
  "$function": {
    "body": <functionCodeString>,
    "args": [ <arg1>, <arg2>, ... ],
    "lang": "js"
  }
}

• body: JavaScript function code as a string
• args: Array of arguments passed to the function
• lang: Must be "js" (Fluxion-safe context)

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✅ Base Example – Sum Two Fields

📥 Input Document

{ "a": 2, "b": 3 }

📌 Expression

{
  "$function": {
    "body": "function(a, b) { return a + b; }",
    "args": ["$a", "$b"],
    "lang": "js"
  }
}

📤 Output

5

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🧱 Ecommerce Example – Calculate Dynamic Discount

📌 Pipeline

[
  {
    "$project": {
      "product": "$name",
      "discountedPrice": {
        "$function": {
          "body": "function(p, d) {
            return Math.round(p - (p * d));
          }",
          "args": ["$price", "$discount"],
          "lang": "js"
        }
      }
    }
  }
]

📥 Input Document

{
  "name": "Speaker",
  "price": 200,
  "discount": 0.15
}

📤 Output

{
  "product": "Speaker",
  "discountedPrice": 170
}

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🔧 Common Use Cases

• Advanced conditional logic
• Complex business formulas
• Processing values beyond built-in operators

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🔗 Related Operators

• $cond, $switch, $reduce, $map

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🧠 Notes

• Fluxion executes $function securely and deterministically.
• Avoid side effects; treat as pure functions.

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