$size operator for array length in MongoDB - Step-by-Step Execution

Concept Flow - $size operator for array length

Start with document

↓

Locate array field

↓

Apply $size operator

↓

Count elements in array

↓

Return count as result

The $size operator counts how many elements are in a specified array field of a document and returns that number.

Execution Sample

MongoDB

db.collection.aggregate([
  { $project: { name: 1, numTags: { $size: "$tags" } } }
])

This query adds a field 'numTags' showing the number of elements in the 'tags' array for each document.

Execution Table

Step	Document	Array Field	Action	Result
1	{ name: "Book A", tags: ["fiction", "bestseller"] }	tags	Apply $size	2
2	{ name: "Book B", tags: ["non-fiction"] }	tags	Apply $size	1
3	{ name: "Book C", tags: [] }	tags	Apply $size	0
4	{ name: "Book D" }	tags	Apply $size	error (field missing)

💡 All documents processed; $size returns count of array elements or error if field missing.

Variable Tracker

Variable	Start	After Doc 1	After Doc 2	After Doc 3	After Doc 4
tags	["fiction", "bestseller"]	["fiction", "bestseller"]	["non-fiction"]	[]	undefined
numTags	undefined	2	1	0	error

Key Moments - 3 Insights

What happens if the array field does not exist in a document?

The $size operator cannot count elements if the array field is missing, resulting in an error, as shown in execution_table row 4.

Does $size count nested arrays or only the top-level array elements?

$size counts only the top-level elements of the specified array field, not elements inside nested arrays.

What if the array is empty?

If the array is empty, $size returns 0, as shown in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the $size result for the document with tags ["fiction", "bestseller"]?

A 1

B 2

C 0

D error

💡 Hint

Check execution_table row 1 under Result column.

At which step does $size return 0?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table rows and find where the array is empty.

If a document has no 'tags' field, what will $size return according to the execution_table?

A error

B 0

C Number of elements in another field

D 1

💡 Hint

See execution_table row 4 Result column.

Concept Snapshot

$size operator counts elements in an array field.
Syntax: { $size: "<arrayField>" }
Returns integer count of array elements.
Returns error if field missing or not an array.
Used in aggregation pipelines to project array length.

Full Transcript

The $size operator in MongoDB counts how many items are inside an array field of a document. When you run an aggregation query with $size, it looks at each document, finds the specified array, counts its elements, and returns that number. If the array is empty, it returns zero. If the array field is missing, it results in an error. This operator is useful to quickly know how many items are in an array without manually counting them.