$round
Definition
Rounds a number to the specified decimal place using ties-to-even rounding (HALF_EVEN); the place defaults to zero.
Expression operator · Arithmetic · Studio Visual: available. Studio validates the complete expression separately.
Syntax
{ "$round": [ <number>, <place> ] }
Replace angle-bracket placeholders with expressions or values.
Optional parameters may be omitted.
Parameters
| Parameter | Required | Description |
|---|---|---|
number |
Yes | Numeric expression. Repeat the argument when the syntax accepts multiple operands. |
place |
No | Decimal position for rounding or truncation. Defaults to 0; negative positions act to the left of the decimal point. |
Arguments are positional: keep the order shown. The authoring signature accepts 1–2 arguments.
Behavior
The following observations use the example expression below. Each row substitutes one argument while retaining the other example arguments. A missing field differs from an explicit null; the result shown is what Core actually returned.
For the number argument:
| Input case | Supplied value | Result |
|---|---|---|
| null | null | null |
| missing | Field absent | null |
| decimal | 2.5 | 2.0 (double) |
| numeric string | "12" | Error — Number and integer precision required |
| text | "hello" | Error — Number and integer precision required |
| boolean | true | Error — Number and integer precision required |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — Number and integer precision required |
| empty array | [] | Error — Number and integer precision required |
| numeric array | [1,2,3] | Error — Number and integer precision required |
| object | {"x":1} | Error — Number and integer precision required |
Argument handling
| Invocation | Result |
|---|---|
| {"$round":[]} | Error — $round requires [number, place?] |
| {"$round":{"unexpected":1}} | Error — Number and integer precision required |
Examples
Expression:
{
"$round": [
2.5
]
}
Input document:
{}
Observed SrotaX result: 2.0 (double).
Detailed input cases
Expand the complete argument-by-argument test matrix
Each table changes only the indicated argument from the example above. Other arguments retain their baseline values. Values normally come from the input field probe; literal configuration arguments are replaced directly. “Missing” means the field is absent, not explicitly null. Numeric strings are strings, not numbers.
Argument: number[0]
| Input case | Supplied value | SrotaX result |
|---|---|---|
| null | null | null |
| missing | Field absent | null |
| zero | 0 | 0 (int32) |
| negative | -2 | -2 (int32) |
| decimal | 2.5 | 2.0 (double) |
| numeric string | "12" | Error — Number and integer precision required |
| text | "hello" | Error — Number and integer precision required |
| empty string | "" | Error — Number and integer precision required |
| boolean | true | Error — Number and integer precision required |
| false | false | Error — Number and integer precision required |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — Number and integer precision required |
| decimal128 | {"$numberDecimal":"1.25"} | 1.0 (double) |
| empty array | [] | Error — Number and integer precision required |
| numeric array | [1,2,3] | Error — Number and integer precision required |
| mixed array | [1,null,"x",3] | Error — Number and integer precision required |
| nested array | [[1,2],[3]] | Error — Number and integer precision required |
| empty object | {} | Error — Number and integer precision required |
| object | {"x":1} | Error — Number and integer precision required |
| ISO date string | "2026-01-15T15:00:00Z" | Error — Number and integer precision required |
Additional boundary examples
Show 2 tested examples
Expression:
{
"$round": [
2.345,
2
]
}
Input:
{}
SrotaX: 2.34 (double).
Expression:
{
"$round": [
2.5,
0
]
}
Input:
{}
SrotaX: 2.0 (double).
Verification
This page records 24 executed cases. It covers individual argument substitutions, not every cross-product of arguments, locale, daylight-saving transitions, numeric overflow, collation or resource limits. No untested behavior is guaranteed.
Download exact expressions, inputs and SrotaX results. Errors are observed messages, not stable public error codes. The reference generator reads the checked-in execution report; do not edit generated observations by hand.
MongoDB differences
Compared with MongoDB 9.0.4: 7 exact BSON matches, 15 both rejected, 2 value/type/acceptance differences, 0 object-field-order differences.
“Exact” includes numeric BSON types and object key order. A value of 3 as int32 differs from 3.0 as double. Object-field-order differences are reported separately; object key ordering is not enforced by the engine regression baseline. Both rejected does not mean the error codes or messages match. This is finite-case evidence, not full compatibility certification.
Show observed differences
| Tested case | SrotaX | MongoDB |
|---|---|---|
| number[0] / decimal128 | 1.0 (double) | 1 (decimal128) |
| all / boundary example | 2.34 (double) | 2.35 (double) |