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$zip

The $zip operator merges multiple arrays element-wise into a single array of arrays.

📌 Syntax

{
  "$zip": {
    "inputs": [ <array1>, <array2>, ... ],
    "useLongestLength": <bool>,  // optional
    "defaults": [ <default1>, <default2>, ... ] // optional
  }
}
  • inputs: The arrays to zip together
  • useLongestLength: If true, extends shorter arrays with null or defaults
  • defaults: Values to use when an array runs out of elements

✅ Base Example – Merge Name and Age Lists

📥 Input Document

{
  "names": ["Alice", "Bob", "Charlie"],
  "ages": [30, 25, 35]
}

📌 Expression

{
  "$zip": {
    "inputs": ["$names", "$ages"]
  }
}

📤 Output

[["Alice", 30], ["Bob", 25], ["Charlie", 35]]

✅ With Unequal Length Arrays and Defaults

📥 Input Document

{
  "a": [1, 2],
  "b": ["x"]
}

📌 Expression

{
  "$zip": {
    "inputs": ["$a", "$b"],
    "useLongestLength": true,
    "defaults": [null, "NA"]
  }
}

📤 Output

[[1, "x"], [2, "NA"]]

🧱 Ecommerce Example – Merge Features and Availability

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "product": "$items.name",
      "featuresZipped": {
        "$zip": {
          "inputs": ["$items.features", "$items.available"],
          "useLongestLength": true,
          "defaults": ["Unknown", false]
        }
      }
    }
  }
]

📥 Input Document

{
  "items": [
    {
      "name": "Table",
      "features": ["Wood", "Foldable"],
      "available": [true]
    }
  ]
}

📤 Output

[
  {
    "product": "Table",
    "featuresZipped": [["Wood", true], ["Foldable", false]]
  }
]

🔧 Common Use Cases

  • Merge parallel arrays for display
  • Create structured tabular pairs
  • Fill missing values dynamically
  • $map, $concatArrays, $reduce, $arrayElemAt

🧠 Notes

  • All arrays must be of equal length unless useLongestLength is true.
  • Combine with $map to format zipped output.