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MongoDB Complete Course / Aggregation Advanced

$reduce expression

Learn $reduce expression in MongoDB through the exact construct, a dedicated example, a boundary case, and a mastery check.

12 min read • Intermediate

$reduce expression is an atomic MongoDB lesson inside Aggregation Advanced. The example uses the specific syntax, command, API, instruction, or language rule named by this lesson.

Study the construct itself, then change one relevant input and predict the result before running it. That keeps this lesson focused on $reduce expression rather than on generic MongoDB syntax.

Topic-specific example

mongodb-06-10.js
javascript Try It Live
db.orders.aggregate([{ $project:{ total:{ $reduce:{input:"$values",initialValue:0,in:{$add:["$$value","$$this"]}} } } }])
Edge case / engineering note

Test $reduce expression with an empty result, NULL/missing value, duplicate key, or concurrent operation where relevant; database behavior should be verified at the boundary shown by this example.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content