How to Use $[] All Positional Operator in MongoDB

In MongoDB, the $[] all positional operator lets you update all elements of an array in a document at once. Use it inside an update command with $set or other update operators to apply changes to every array element without specifying each index.

Syntax

The $[] operator is used inside update statements to target all elements of an array field. It is placed inside the array field name in the update document.

• { $set: { 'arrayField.$[]': value } } updates all elements in arrayField to value.
• $[] works only with update operators like $set, $inc, etc.

json

{
  "$set": {
    "arrayField.$[]": <newValue>
  }
}

Example

This example updates all elements in the scores array by adding 5 to each score.

mongodb

db.students.updateMany(
  {},
  { $inc: { "scores.$[]": 5 } }
);

Output

{ "acknowledged" : true, "matchedCount" : 2, "modifiedCount" : 2 }

Common Pitfalls

Common mistakes when using $[] include:

• Trying to use $[] outside of update operators like $set or $inc.
• Using $[] on fields that are not arrays, which causes errors.
• Confusing $[] with the single positional operator $, which updates only the first matching element.

mongodb

/* Wrong: $[] used without update operator */
db.collection.updateMany({}, { "arrayField.$[]": 10 });

/* Right: $[] used with $set */
db.collection.updateMany({}, { $set: { "arrayField.$[]": 10 } });

Quick Reference

• $[]: Targets all elements in an array.
• Use only inside update operators like $set, $inc, $mul, etc.
• Works only on array fields.
• Different from $ which targets the first matching element.

Key Takeaways

Use $[] to update all elements in an array field in MongoDB documents.

$[] must be used inside update operators like $set or $inc.

Do not use $[] on non-array fields or outside update operators.

$[] updates every element, unlike $ which updates only the first matched element.