MongoDB $where Operator

What Is $where?

$where is a MongoDB query operator that runs a JavaScript expression on each document in find(). Keep the document when the expression returns true. Inside the script, this is the current document. For new work, prefer $expr.

$where = JavaScript filter for find()

{ $where: "this.qty * this.price > 1000" }
{ $where: function() { return this.qty * this.price > 1000; } }

this → current document
return true → keep document

Prefer today:
{ $expr: { $gt: [ { $multiply: ["$qty","$price"] }, 1000 ] } }

Slow · hard to index · never build from user input

How to Use $where in MongoDB

Write a boolean JavaScript check, pass it to $where, then prefer rewriting it with $expr.

1. Start from a find() filter (not an aggregation expression stage).
2. Write a string or function that reads this.field and returns true/false.
3. Test on a small collection so you can see which rows match.
4. Rewrite the same logic with $expr + native operators for production.

Pseudocode

for each document in collection:
    this = document
    ok = evaluate(javascriptExpression)   // must be boolean
    if ok is true:
        keep document
    else:
        skip document

Cheat Sheet

Keep these facts handy while reading or replacing $where filters.

Live Preview

Simulate $where: "this.qty * this.price > threshold" on the sample products. Change the threshold and run.

$where: "this.qty * this.price > 1000"

Widget A: 50 * 25 = 1250 → keep
Widget B: 10 * 120 = 1200 → keep
Widget C: 100 * 8.5 = 850 → skip
Widget D: 5 * 250 = 1250 → keep

matched: 3

Worked Walkthrough — Inventory Value

One product has qty: 50 and price: 25. Goal: keep it only if inventory value is greater than 1000.

Document:
  { name: "Widget A", qty: 50, price: 25 }

$where: "this.qty * this.price > 1000"

this.qty * this.price
  = 50 * 25
  = 1250

1250 > 1000 → true → keep document

Same idea with $expr (preferred):
  $multiply(["$qty","$price"]) → 1250
  $gt([1250, 1000]) → true

MongoDB Examples

We use a small products collection. First filter with $where, then rewrite the same rule with $expr. Run these in mongosh after inserting the sample data once.

db.products.insertMany([
  { _id: 1, name: "Widget A", qty: 50, price: 25 },
  { _id: 2, name: "Widget B", qty: 10, price: 120 },
  { _id: 3, name: "Widget C", qty: 100, price: 8.5 },
  { _id: 4, name: "Widget D", qty: 5, price: 250 }
]);

db.products.find()

[
  { _id: 1, name: "Widget A", qty: 50, price: 25 },
  { _id: 2, name: "Widget B", qty: 10, price: 120 },
  { _id: 3, name: "Widget C", qty: 100, price: 8.5 },
  { _id: 4, name: "Widget D", qty: 5, price: 250 }
]

db.products.find({
  $where: "this.qty * this.price > 1000"
});

{ _id: 1, name: "Widget A", qty: 50, price: 25 }
{ _id: 2, name: "Widget B", qty: 10, price: 120 }
{ _id: 4, name: "Widget D", qty: 5, price: 250 }

db.products.find({
  $expr: {
    $gt: [
      { $multiply: ["$qty", "$price"] },
      1000
    ]
  }
});

{ _id: 1, name: "Widget A", qty: 50, price: 25 }
{ _id: 2, name: "Widget B", qty: 10, price: 120 }
{ _id: 4, name: "Widget D", qty: 5, price: 250 }

Performance Notes

Narrow the set first with indexed filters when possible, then apply expression logic. Avoid $where on large collections.

Key Takeaways