Bending Arrays to Your Will: $map, $filter, $reduce

Welcome to Day 4! 🧬

You often store data in arrays (e.g., shopping cart items, tag scores). Sometimes you need to modify those arrays on the fly before sending them to the client. Do NOT loop in Node.js. Use MongoDB Operators.

1. $map: The Transformation

Works exactly like Array.prototype.map() in JS.

Scenario: You have prices in USD. You want to add a field showing prices in EUR (x0.85).

db.orders.aggregate([
  {
    $project: {
      items: {
        $map: {
          input: "$items",
          as: "item", // Variable name
          in: {
            name: "$$item.name",
            priceUSD: "$$item.price",
            priceEUR: { $multiply: ["$$item.price", 0.85] }
          }
        }
      }
    }
  }
])

Note the double dollar sign $$ to refer to the variable defined in as.

2. $filter: The Cleaner

Works like Array.prototype.filter().

Scenario: You have an array of scores. You only want to show scores > 80.

db.students.aggregate([
  {
    $project: {
      highScores: {
        $filter: {
          input: "$scores",
          as: "score",
          cond: { $gt: ["$$score", 80] }
        }
      }
    }
  }
])

3. $reduce: The Aggregator

Works like Array.prototype.reduce().

Scenario: You have an array of objects { type: "A", value: 10 }. You want to sum up values but only for type A.

db.data.aggregate([
  {
    $project: {
      totalTypeA: {
        $reduce: {
          input: "$data",
          initialValue: 0,
          in: {
            $cond: {
              if: { $eq: ["$$this.type", "A"] },
              then: { $add: ["$$value", "$$this.value"] },
              else: "$$value"
            }
          }
        }
      }
    }
  }
])

$$value is the accumulator. $$this is the current element.

4. $concatArrays: The Merger

Joins multiple arrays into one.

{ $project: { allTags: { $concatArrays: ["$tags1", "$tags2"] } } }

🧠 Daily Challenge

You have a Cart with items: [{ name: "Apple", price: 10, discount: true }, { name: "Video Game", price: 50, discount: false }].

1. Use $filter to get only discounted items.
2. Use $map to apply a 10% off label to them.
3. Use $reduce to calculate the total checkout price.

See you on Day 5 for our Mini Project: Building an Analytics Dashboard! 📊

Next Step

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