$reduce
Reference group: Array aggregation expressions
What it does
Folds an array into one result using $$value and $$this.
Syntax
{$reduce: { input, initialValue, in }}
Use case
Compute totals or custom accumulations.
$reduce: practical example
{ $project: { total: { $reduce: { input: "$items", initialValue: 0, in: { $add: ["$$value", { $multiply: ["$$this.price", "$$this.qty"] }] } } } } }💡 Explanation
Folds an array into one result using
$$value and $$this.
When to use it
Compute totals or custom accumulations.Key Takeaways
- Folds an array into one result using
$$valueand$$this. - Typical use: Compute totals or custom accumulations.