An "annotated" aggregation expression showing intermediate values at everly level of the expression tree

debugger.json

{
  "$reduce": {                    // 300
    "input": {                      // [2, 4, 6, 8, 10]
      "$map": {                       // [2, 4, 6, 8, 10]
        "input": {                      // [2, 4, 6, 8, 10]
          "$filter": {                    // [2, 4, 6, 8, 10]
            "input": "$numList",            // [1, 2, 3, 4, 5, 6, 7, 8, 9. 10]
            "cond": {
              "$eq": [                        // [false, true, false, true, false, true, false, true, false, true]
                {
                  "$mod": [                     // [1, 0, 1, 0, 1, 0, 1, 0, 1, 0]
                    "$$this",
                    2                             // 2
                  ]
                },
                0                              // 0
              ]
            }
          }
        },
        "in": {                         // [20, 40, 60, 80, 100]
          "$multiply": [                  // [20, 40, 60, 80, 100]
            "$$this",
            10
          ]
        }
      }
    },
    "initialValue": 0,          // 0
    "in": {                     // 300
      "$add": [                   // [0, 20, 60, 120, 200, 300]
        "$$value",
        "$$this"
      ]
    }
  }
}