C100DEV Aggregation Framework Practice Question
Which operator is most appropriate to concatenate an array of strings into a single string field?
⚠ Common exam trap
Candidates often attempt to use $concat on an array directly, which causes an error because $concat expects individual string arguments, not an array of strings.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
$reduce
The $reduce operator provides a way to iterate through an array and apply an expression to each element, allowing you to build a cumulative result. When used with $concat, it is the standard and most performant way to join array elements into a single string. This is common when normalizing document data for export or display purposes in external applications.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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$map
Why it's wrong here
$map is used to transform each element of an array, not to aggregate them into a single value. It returns an array of the same length as the input, which is not suitable for collapsing an array of strings into a single consolidated string value.
- ✗
$concat
Why it's wrong here
While $concat performs string concatenation, it only accepts a list of string arguments. It does not natively iterate over an array; therefore, it cannot process an array of arbitrary length unless you use it in conjunction with an iterator like $reduce.
- ✓
$reduce
Why this is correct
$reduce allows you to iterate over an array and build a result. By using $concat as the accumulation expression, you can effectively join an array of strings into a single string. It is the most robust way to perform array-to-scalar transformations within the aggregation pipeline.
- ✗
$filter
Why it's wrong here
$filter is designed to select a subset of an array based on a condition. It does not perform string manipulation or concatenation, making it entirely inappropriate for the task of combining multiple string elements into a single, cohesive string output.
About these practice questions
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Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official MongoDB exam blueprint
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