# # `sum()`

The sum method returns the sum of all items in the collection:

    collect([1, 2, 3]).sum();

    //=> 6

If the collection contains nested arrays or objects, you should pass a key to use for determining which values to sum:

    const collection = collect([
      { name: "JavaScript: The Good Parts", pages: 176 },
      { name: "JavaScript: The Definitive Guide", pages: 1096 }
    ]);

    collection.sum("pages");

    //=> 1272

In addition, you may pass your own callback to determine which values of the collection to sum:

    const collection = collect([
      { name: "Desk", colors: ["Black", "Mahogany"] },
      { name: "Chair", colors: ["Black"] },
      { name: "Bookcase", colors: ["Red", "Beige", "Brown"] }
    ]);

    const total = collection.sum(function(product) {
      return product.colors.length;
    });

    //=> 6

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