$log
Definition
Computes a logarithm using the supplied base.
Expression operator · Arithmetic · Studio Visual: available. Studio validates the complete expression separately.
Syntax
{ "$log": [ <left>, <right> ] }
Replace angle-bracket placeholders with expressions or values.
Optional parameters may be omitted.
Parameters
| Parameter | Required | Description |
|---|---|---|
left |
Yes | Number whose logarithm is calculated. |
right |
Yes | Logarithm base. |
Arguments are positional: keep the order shown. The authoring signature accepts 2–2 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 left argument:
| Input case | Supplied value | Result |
|---|---|---|
| null | null | Error — $log number must resolve to a numeric value |
| missing | Field absent | Error — $log number must resolve to a numeric value |
| decimal | 2.5 | 1.3219280948873624 (double) |
| numeric string | "12" | Error — $log number must resolve to a numeric value |
| text | "hello" | Error — $log number must resolve to a numeric value |
| boolean | true | Error — $log number must resolve to a numeric value |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — $log number must resolve to a numeric value |
| empty array | [] | Error — $log number must resolve to a numeric value |
| numeric array | [1,2,3] | Error — $log number must resolve to a numeric value |
| object | {"x":1} | Error — $log number must resolve to a numeric value |
Argument handling
| Invocation | Result |
|---|---|
| {"$log":[]} | Error — $log expects [number, base] |
| {"$log":{"unexpected":1}} | Error — $log expects [number, base] |
Examples
Expression:
{
"$log": [
8,
2
]
}
Input document:
{}
Observed SrotaX result: 3.0 (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: left[0]
| Input case | Supplied value | SrotaX result |
|---|---|---|
| null | null | Error — $log number must resolve to a numeric value |
| missing | Field absent | Error — $log number must resolve to a numeric value |
| zero | 0 | Error — $log requires positive number and base != 1 |
| negative | -2 | Error — $log requires positive number and base != 1 |
| decimal | 2.5 | 1.3219280948873624 (double) |
| numeric string | "12" | Error — $log number must resolve to a numeric value |
| text | "hello" | Error — $log number must resolve to a numeric value |
| empty string | "" | Error — $log number must resolve to a numeric value |
| boolean | true | Error — $log number must resolve to a numeric value |
| false | false | Error — $log number must resolve to a numeric value |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — $log number must resolve to a numeric value |
| decimal128 | {"$numberDecimal":"1.25"} | 0.32192809488736235 (double) |
| empty array | [] | Error — $log number must resolve to a numeric value |
| numeric array | [1,2,3] | Error — $log number must resolve to a numeric value |
| mixed array | [1,null,"x",3] | Error — $log number must resolve to a numeric value |
| nested array | [[1,2],[3]] | Error — $log number must resolve to a numeric value |
| empty object | {} | Error — $log number must resolve to a numeric value |
| object | {"x":1} | Error — $log number must resolve to a numeric value |
| ISO date string | "2026-01-15T15:00:00Z" | Error — $log number must resolve to a numeric value |
Argument: right[1]
| Input case | Supplied value | SrotaX result |
|---|---|---|
| null | null | Error — $log base must resolve to a numeric value |
| missing | Field absent | Error — $log base must resolve to a numeric value |
| zero | 0 | Error — $log requires positive number and base != 1 |
| negative | -2 | Error — $log requires positive number and base != 1 |
| decimal | 2.5 | 2.26941239209809 (double) |
| numeric string | "12" | Error — $log base must resolve to a numeric value |
| text | "hello" | Error — $log base must resolve to a numeric value |
| empty string | "" | Error — $log base must resolve to a numeric value |
| boolean | true | Error — $log base must resolve to a numeric value |
| false | false | Error — $log base must resolve to a numeric value |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — $log base must resolve to a numeric value |
| decimal128 | {"$numberDecimal":"1.25"} | 9.318851158516168 (double) |
| empty array | [] | Error — $log base must resolve to a numeric value |
| numeric array | [1,2,3] | Error — $log base must resolve to a numeric value |
| mixed array | [1,null,"x",3] | Error — $log base must resolve to a numeric value |
| nested array | [[1,2],[3]] | Error — $log base must resolve to a numeric value |
| empty object | {} | Error — $log base must resolve to a numeric value |
| object | {"x":1} | Error — $log base must resolve to a numeric value |
| ISO date string | "2026-01-15T15:00:00Z" | Error — $log base must resolve to a numeric value |
Verification
This page records 41 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: 3 exact BSON matches, 32 both rejected, 6 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 |
|---|---|---|
| left[0] / null | Error — $log number must resolve to a numeric value | null |
| left[0] / missing | Error — $log number must resolve to a numeric value | null |
| left[0] / decimal128 | 0.32192809488736235 (double) | 0.3219280948873623478703194294893902 (decimal128) |
| right[1] / null | Error — $log base must resolve to a numeric value | null |
| right[1] / missing | Error — $log base must resolve to a numeric value | null |
| right[1] / decimal128 | 9.318851158516168 (double) | 9.318851158516169628007195475350726 (decimal128) |