$push, $addToSet and $pull

Three operators cover most array writes, and the only one people confuse is $push versus $addToSet. Both append; only one checks whether the value is already there.

Given:

{
  _id: 1,
  skills: ["Node.js"]
}

Add

$push: {
  skills: "MongoDB"
}

Result:

["Node.js", "MongoDB"]

Prevent duplicates

$addToSet: {
  skills: "MongoDB"
}

If MongoDB already exists:

Nothing happens.

This distinction is important:

$push       → always adds

$addToSet   → adds only if not already present

Remove from array

$pull: {
  skills: "MongoDB"
}

Try it

User 101 starts with ["Node.js", "Kafka"]:

db.users.updateOne(
  { _id: 101 },
  {
    $addToSet: { skills: "MongoDB" }
  }
)

Run it twice: the second time modifiedCount is 0, because the value is already there. Swap $addToSet for $push and it gets added again.