The increment operator ($inc) is used to increment the value of a numeric field by a specific number.

The operator accepts a document containing pairs of a field name and a number.

Given a positive number, the value of the field will be incremented and if a negative number is provided, the value will be decremented.

It is obvious but worth mentioning that the $inc operator can only be used with numeric fields; if attempted for non-numeric fields, the operation fails with an error.

Currently, in our products collection, the document for a product looks as shown here:

_id:ObjectId("61a32cf04a960cdc89a35a89")
name:"Airpods Pro"
brand:"Apple"
category:"Electronics"
price:99.99
soldQty:240
__v:0

Now, we will write an update using the $inc operator on the soldQty field:

const updateProduct = async () => {
  const result = await Product.findOneAndUpdate(
    {
      name: "Airpods Pro",
    },
    {
      $inc: { soldQty: 2 },
    },
    { returnNewDocument: true }
  )
  console.log(result)
}

updateProduct()

The preceding update operation finds a product by its name and increments the soldQty field by 2. It also sets returnNewDocument to true, so that the updated record will be returned in the response. You can see the output below:

{
   _id: new ObjectId("61a32cf04a960cdc89a35a89"),
   name: 'Airpods Pro',
   brand: 'Apple',
   category: 'Electronics',
   price: 305.7,
   soldQty: 242,
   __v: 0
}

We can see that the update command was successful - the soldQty field is correctly incremented by 2