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$filter

The $filter operator selects elements from an array that satisfy a condition.

📌 Syntax

{
  "$filter": {
    "input": <arrayExpression>,
    "as": <variableName>,
    "cond": <booleanExpression>
  }
}
  • input: The array to filter
  • as: The variable name for each item
  • cond: The condition that must be true to include the item

✅ Base Example – Filter Even Numbers

📥 Input Document

{ "nums": [1, 2, 3, 4, 5, 6] }

📌 Expression

{
  "$filter": {
    "input": "$nums",
    "as": "num",
    "cond": { "$eq": [{ "$mod": ["$$num", 2] }, 0] }
  }
}

📤 Output

[2, 4, 6]

✅ Base Example – Remove Empty Strings

📥 Input Document

{ "tags": ["", "sale", null, "new", ""] }

📌 Expression

{
  "$filter": {
    "input": "$tags",
    "as": "tag",
    "cond": { "$and": [{ "$ne": ["$$tag", ""] }, { "$ne": ["$$tag", null] }] }
  }
}

📤 Output

["sale", "new"]

🧱 Ecommerce Example – Filter High-Value Items

📌 Pipeline

[
  {
    "$project": {
      "items": {
        "$filter": {
          "input": "$items",
          "as": "item",
          "cond": { "$gt": ["$$item.price", 100] }
        }
      }
    }
  }
]

📥 Input Document

{
  "items": [
    { "name": "Book", "price": 40 },
    { "name": "Tablet", "price": 300 },
    { "name": "Monitor", "price": 150 }
  ]
}

📤 Output

{
  "items": [
    { "name": "Tablet", "price": 300 },
    { "name": "Monitor", "price": 150 }
  ]
}

🔧 Common Use Cases

  • Select items matching a filter
  • Clean arrays of null/empty/invalid entries
  • Apply logic to sub-documents or features
  • $map, $reduce, $anyElementTrue, $allElementsTrue

🧠 Notes

  • Use $$ to access loop variables.
  • Useful for filtering ecommerce features, specs, or product tags.