How to Use $inc Operator in MongoDB for Incrementing Values

In MongoDB, use the $inc operator to increase or decrease the value of a numeric field in a document. It modifies the field by the specified amount without replacing the whole document. For example, { $inc: { score: 1 } } adds 1 to the current value of score.

Syntax

The $inc operator takes an object where keys are field names and values are the amounts to increment or decrement. Positive values increase the field, negative values decrease it.

• Field: The numeric field to update.
• Value: The number to add (can be negative to subtract).

mongodb

db.collection.updateOne(
  { <filter> },
  { $inc: { <field1>: <amount1>, <field2>: <amount2>, ... } }
)

Example

This example shows how to increment the views field by 1 in a document where _id is 1.

mongodb

db.articles.updateOne(
  { _id: 1 },
  { $inc: { views: 1 } }
)

// To decrement views by 2:
db.articles.updateOne(
  { _id: 1 },
  { $inc: { views: -2 } }
)

Output

{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

Common Pitfalls

Common mistakes when using $inc include:

• Trying to increment a field that is not numeric, which causes an error.
• Using $inc on a field that does not exist will create the field with the increment value.
• Passing a non-numeric value to $inc will fail.

Always ensure the field is numeric or absent before using $inc.

mongodb

/* Wrong: incrementing a string field */
db.users.updateOne(
  { _id: 1 },
  { $inc: { name: 1 } } // Error: cannot apply $inc to string
)

/* Correct: incrementing a numeric field */
db.users.updateOne(
  { _id: 1 },
  { $inc: { age: 1 } }
)

Quick Reference

Usage	Description
{ $inc: { field: 1 } }	Increment field by 1
{ $inc: { field: -1 } }	Decrement field by 1
{ $inc: { field1: 2, field2: -3 } }	Increment field1 by 2 and decrement field2 by 3
Using $inc on non-existing field	Creates the field with the increment value

Key Takeaways

Use $inc to add or subtract numeric values in a document field without replacing the whole document.

$inc works only on numeric fields; applying it to non-numeric fields causes errors.

If the field does not exist, $inc creates it with the increment value.

You can increment multiple fields at once by specifying them inside $inc.

Always verify the field type before using $inc to avoid update failures.