Aggregation Pipeline Operators
- Expression Operators
- Alphabetical Listing of Expression Operators

Aggregation Pipeline Operators

Note

For details on specific operator, including syntax and examples,click on the specific operator to go to its reference page.

Expression Operators

These expression operators are available to construct expressions for use in the aggregation pipelinestages.

Operator expressions are similar to functions that take arguments.In general, these expressions take an array of argumentsand have the following form:

{ <operator>: [ <argument1>, <argument2> ... ] }

If operator accepts a single argument, you can omit the outer arraydesignating the argument list:

{ <operator>: <argument> }

To avoid parsing ambiguity if the argument is a literal array, you mustwrap the literal array in a $literal expression or keepthe outer array that designates the argument list.

Arithmetic Expression Operators

Arithmetic expressions perform mathematic operations on numbers. Somearithmetic expressions can also support date arithmetic.

Name	Description
`$abs`	Returns the absolute value of a number.
`$add`	Adds numbers to return the sum, or adds numbers and a date toreturn a new date. If adding numbers and a date, treats thenumbers as milliseconds. Accepts any number of argumentexpressions, but at most, one expression can resolve to adate.
`$ceil`	Returns the smallest integer greater than or equal to the specified number.
`$divide`	Returns the result of dividing the first number by thesecond. Accepts two argument expressions.
`$exp`	Raises e to the specified exponent.
`$floor`	Returns the largest integer less than or equal to thespecified number.
`$ln`	Calculates the natural log of a number.
`$log`	Calculates the log of a number in the specified base.
`$log10`	Calculates the log base 10 of a number.
`$mod`	Returns the remainder of the first number divided by thesecond. Accepts two argument expressions.
`$multiply`	Multiplies numbers to return the product. Accepts any numberof argument expressions.
`$pow`	Raises a number to the specified exponent.
`$round`	Rounds a number to to a whole integer or to a specifieddecimal place.
`$sqrt`	Calculates the square root.
`$subtract`	Returns the result of subtracting the second value from thefirst. If the two values are numbers, return the difference.If the two values are dates, return the difference inmilliseconds. If the two values are a date and a number inmilliseconds, return the resulting date. Accepts two argumentexpressions. If the two values are a date and a number,specify the date argument first as it is not meaningful tosubtract a date from a number.
`$trunc`	Truncates a number to a whole integer or to a specifieddecimal place.

Array Expression Operators

`$arrayElemAt`	Returns the element at the specified array index.
`$arrayToObject`	Converts an array of key value pairs to a document.
`$concatArrays`	Concatenates arrays to return the concatenated array.
`$filter`	Selects a subset of the array to return an array with onlythe elements that match the filter condition.
`$in`	Returns a boolean indicating whether a specified value is inan array.
`$indexOfArray`	Searches an array for an occurence of a specified value andreturns the array index of the first occurence. If thesubstring is not found, returns `-1`.
`$isArray`	Determines if the operand is an array. Returns a boolean.
`$map`	Applies a subexpression to each element of an array andreturns the array of resulting values in order. Accepts namedparameters.
`$objectToArray`	Converts a document to an array of documents representingkey-value pairs.
`$range`	Outputs an array containing a sequence of integers accordingto user-defined inputs.
`$reduce`	Applies an expression to each element in an array andcombines them into a single value.
`$reverseArray`	Returns an array with the elements in reverse order.
`$size`	Returns the number of elements in the array. Accepts a singleexpression as argument.
`$slice`	Returns a subset of an array.
`$zip`	Merge two arrays together.

Boolean Expression Operators

Boolean expressions evaluate their argument expressions as booleans andreturn a boolean as the result.

In addition to the false boolean value, Boolean expression evaluatesas false the following: null, 0, and undefinedvalues. The Boolean expression evaluates all other values as true,including non-zero numeric values and arrays.

Name	Description
`$and`	Returns `true` only when all its expressions evaluate to`true`. Accepts any number of argument expressions.
`$not`	Returns the boolean value that is the opposite of itsargument expression. Accepts a single argument expression.
`$or`	Returns `true` when any of its expressions evaluates to`true`. Accepts any number of argument expressions.

Comparison Expression Operators

Comparison expressions return a boolean except for $cmpwhich returns a number.

The comparison expressions take two argument expressions and compareboth value and type, using the specified BSON comparison order for values of different types.

`$cmp`	Returns `0` if the two values are equivalent, `1` if thefirst value is greater than the second, and `-1` if thefirst value is less than the second.
`$eq`	Returns `true` if the values are equivalent.
`$gt`	Returns `true` if the first value is greater than thesecond.
`$gte`	Returns `true` if the first value is greater than or equalto the second.
`$lt`	Returns `true` if the first value is less than the second.
`$lte`	Returns `true` if the first value is less than or equal tothe second.
`$ne`	Returns `true` if the values are not equivalent.

Conditional Expression Operators

Name	Description
`$cond`	A ternary operator that evaluates one expression, anddepending on the result, returns the value of one of theother two expressions. Accepts either three expressions in anordered list or three named parameters.
`$ifNull`	Returns either the non-null result of the first expression orthe result of the second expression if the first expressionresults in a null result. Null result encompasses instancesof undefined values or missing fields. Accepts twoexpressions as arguments. The result of the second expressioncan be null.
`$switch`	Evaluates a series of case expressions. When it finds anexpression which evaluates to `true`, `$switch` executesa specified expression and breaks out of the control flow.

Date Expression Operators

The following operators returns date objects or components of a dateobject:

Name	Description
`$dateFromParts`	Constructs a BSON Date object given the date’s constituentparts.
`$dateFromString`	Converts a date/time string to a date object.
`$dateToParts`	Returns a document containing the constituent parts of a date.
`$dateToString`	Returns the date as a formatted string.
`$dayOfMonth`	Returns the day of the month for a date as a number between 1and 31.
`$dayOfWeek`	Returns the day of the week for a date as a number between 1(Sunday) and 7 (Saturday).
`$dayOfYear`	Returns the day of the year for a date as a number between 1and 366 (leap year).
`$hour`	Returns the hour for a date as a number between 0 and 23.
`$isoDayOfWeek`	Returns the weekday number in ISO 8601 format, ranging from`1` (for Monday) to `7` (for Sunday).
`$isoWeek`	Returns the week number in ISO 8601 format, ranging from`1` to `53`. Week numbers start at `1` with the week(Monday through Sunday) that contains the year’s firstThursday.
`$isoWeekYear`	Returns the year number in ISO 8601 format. The year startswith the Monday of week 1 (ISO 8601) and ends with the Sundayof the last week (ISO 8601).
`$millisecond`	Returns the milliseconds of a date as a number between 0 and999.
`$minute`	Returns the minute for a date as a number between 0 and 59.
`$month`	Returns the month for a date as a number between 1 (January)and 12 (December).
`$second`	Returns the seconds for a date as a number between 0 and 60(leap seconds).
`$toDate`	Converts value to a Date.New in version 4.0.
`$week`	Returns the week number for a date as a number between 0 (thepartial week that precedes the first Sunday of the year) and53 (leap year).
`$year`	Returns the year for a date as a number (e.g. 2014).

The following arithmetic operators can take date operands:

Name	Description
`$add`	Adds numbers and a date to return a new date. If addingnumbers and a date, treats the numbers as milliseconds.Accepts any number of argument expressions, but at most, oneexpression can resolve to a date.
`$subtract`	Returns the result of subtracting the second value from thefirst. If the two values are dates, return the difference inmilliseconds. If the two values are a date and a number inmilliseconds, return the resulting date. Accepts two argumentexpressions. If the two values are a date and a number,specify the date argument first as it is not meaningful tosubtract a date from a number.

Literal Expression Operator

Name	Description
`$literal`	Return a value without parsing. Use for values that theaggregation pipeline may interpret as an expression. Forexample, use a `$literal` expression to a stringthat starts with a `$` to avoid parsing as a field path.

Object Expression Operators

Name	Description
`$mergeObjects`	Combines multiple documents into a single document.New in version 3.6.
`$objectToArray`	Converts a document to an array of documents representingkey-value pairs.New in version 3.6.

Set Expression Operators

Set expressions performs set operation on arrays, treating arrays assets. Set expressions ignores the duplicate entries in each input arrayand the order of the elements.

If the set operation returns a set, the operation filters outduplicates in the result to output an array that contains only uniqueentries. The order of the elements in the output array is unspecified.

If a set contains a nested array element, the set expression does not descendinto the nested array but evaluates the array at top-level.

Name	Description
`$allElementsTrue`	Returns `true` if no element of a set evaluates to`false`, otherwise, returns `false`. Accepts a singleargument expression.
`$anyElementTrue`	Returns `true` if any elements of a set evaluate to`true`; otherwise, returns `false`. Accepts a singleargument expression.
`$setDifference`	Returns a set with elements that appear in the first setbut not in the second set; i.e. performs a relativecomplement)of the second set relative to the first. Accepts exactlytwo argument expressions.
`$setEquals`	Returns `true` if the input sets have the same distinctelements. Accepts two or more argument expressions.
`$setIntersection`	Returns a set with elements that appear in all of theinput sets. Accepts any number of argument expressions.
`$setIsSubset`	Returns `true` if all elements of the first set appearin the second set, including when the first set equals thesecond set; i.e. not a strict subset. Accepts exactlytwo argument expressions.
`$setUnion`	Returns a set with elements that appear in any of theinput sets.

String Expression Operators

String expressions, with the exception of$concat, only have a well-defined behavior for strings of ASCII characters.

$concat behavior is well-defined regardless ofthe characters used.

Name	Description
`$concat`	Concatenates any number of strings.
`$dateFromString`	Converts a date/time string to a date object.
`$dateToString`	Returns the date as a formatted string.
`$indexOfBytes`	Searches a string for an occurence of a substring and returnsthe UTF-8 byte index of the first occurence. If the substringis not found, returns `-1`.
`$indexOfCP`	Searches a string for an occurence of a substring and returnsthe UTF-8 code point index of the first occurence. If thesubstring is not found, returns `-1`
`$ltrim`	Removes whitespace or the specified characters from thebeginning of a string.New in version 4.0.
`$regexFind`	Applies a regular expression (regex) to a string and returnsinformation on the first matched substring.New in version 4.2.
`$regexFindAll`	Applies a regular expression (regex) to a string and returnsinformation on the all matched substrings.New in version 4.2.
`$regexMatch`	Applies a regular expression (regex) to a string and returnsa boolean that indicates if a match is found or not.New in version 4.2.
`$rtrim`	Removes whitespace or the specified characters from theend of a string.New in version 4.0.
`$split`	Splits a string into substrings based on a delimiter. Returnsan array of substrings. If the delimiter is not found withinthe string, returns an array containing the original string.
`$strLenBytes`	Returns the number of UTF-8 encoded bytes in a string.
`$strLenCP`	Returns the number of UTF-8 code points in a string.
`$strcasecmp`	Performs case-insensitive string comparison and returns:`0` if two strings are equivalent, `1` if the firststring is greater than the second, and `-1` if the firststring is less than the second.
`$substr`	Deprecated. Use `$substrBytes` or`$substrCP`.
`$substrBytes`	Returns the substring of a string. Starts with the characterat the specified UTF-8 byte index (zero-based) in the stringand continues for the specified number of bytes.
`$substrCP`	Returns the substring of a string. Starts with the characterat the specified UTF-8 code point (CP) index(zero-based) in the string and continues for the number ofcode points specified.
`$toLower`	Converts a string to lowercase. Accepts a single argumentexpression.
`$toString`	Converts value to a string.New in version 4.0.
`$trim`	Removes whitespace or the specified characters from thebeginning and end of a string.New in version 4.0.
`$toUpper`	Converts a string to uppercase. Accepts a single argumentexpression.

Text Expression Operator

Name	Description
`$meta`	Access text search metadata.

Trigonometry Expression Operators

Trigonometry expressions perform trigonometric operations on numbers.Values that represent angles are always input or output in radians. Use$degreesToRadians and $radiansToDegrees toconvert between degree and radian measurements.

Name	Description
`$sin`	Returns the sine of a value that is measured in radians.
`$cos`	Returns the cosine of a value that is measured in radians.
`$tan`	Returns the tangent of a value that is measured in radians.
`$asin`	Returns the inverse sin (arc sine) of a value in radians.
`$acos`	Returns the inverse cosine (arc cosine) of a value in radians.
`$atan`	Returns the inverse tangent (arc tangent) of a value inradians.
`$atan2`	Returns the inverse tangent (arc tangent) of `y / x` inradians, where `y` and `x` are the first and secondvalues passed to the expression respectively.
`$asinh`	Returns the inverse hyperbolic sine (hyperbolic arc sine) of avalue in radians.
`$acosh`	Returns the inverse hyperbolic cosine (hyperbolic arc cosine)of a value in radians.
`$atanh`	Returns the inverse hyperbolic tangent (hyperbolic arctangent) of a value in radians.
`$degreesToRadians`	Converts a value from degrees to radians.
`$radiansToDegrees`	Converts a value from radians to degrees.

Type Expression Operators

Name	Description
`$convert`	Converts a value to a specified type.New in version 4.0.
`$toBool`	Converts value to a boolean.New in version 4.0.
`$toDate`	Converts value to a Date.New in version 4.0.
`$toDecimal`	Converts value to a Decimal128.New in version 4.0.
`$toDouble`	Converts value to a double.New in version 4.0.
`$toInt`	Converts value to an integer.New in version 4.0.
`$toLong`	Converts value to a long.New in version 4.0.
`$toObjectId`	Converts value to an ObjectId.New in version 4.0.
`$toString`	Converts value to a string.New in version 4.0.
`$type`	Return the BSON data type of the field.

Accumulators ($group)

Available for use in the $group stage, accumulators areoperators that maintain their state (e.g. totals, maximums, minimums,and related data) as documents progress through the pipeline.

When used as accumulators in the $group stage, theseoperators take as input a single expression, evaluating the expressiononce for each input document, and maintain their stage for the group ofdocuments that share the same group key.

Name	Description
`$addToSet`	Returns an array of unique expression values for eachgroup. Order of the array elements is undefined.
`$avg`	Returns an average of numerical values. Ignores non-numeric values.
`$first`	Returns a value from the first document for each group. Orderis only defined if the documents are in a defined order.
`$last`	Returns a value from the last document for each group. Orderis only defined if the documents are in a defined order.
`$max`	Returns the highest expression value for each group.
`$mergeObjects`	Returns a document created by combining the input documentsfor each group.
`$min`	Returns the lowest expression value for each group.
`$push`	Returns an array of expression values for each group.
`$stdDevPop`	Returns the population standard deviation of the input values.
`$stdDevSamp`	Returns the sample standard deviation of the input values.
`$sum`	Returns a sum of numerical values. Ignores non-numeric values.

Accumulators (in Other Stages)

Some operators that are available as accumulators for the$group stage are also available for use in other stages butnot as accumulators. When used in these other stages, these operatorsdo not maintain their state and can take as input either a singleargument or multiple arguments. For details, refer to the specificoperator page.

Changed in version 3.2.

The following accumulator operators are also available in the$project, $addFields, and $set stages.

Name	Description
`$avg`	Returns an average of the specified expression or list ofexpressions for each document. Ignores non-numeric values.
`$max`	Returns the maximum of the specified expression or list ofexpressions for each document
`$min`	Returns the minimum of the specified expression or list ofexpressions for each document
`$stdDevPop`	Returns the population standard deviation of the input values.
`$stdDevSamp`	Returns the sample standard deviation of the input values.
`$sum`	Returns a sum of numerical values. Ignores non-numeric values.

Variable Expression Operators

Name	Description
`$let`	Defines variables for use within the scope of a subexpressionand returns the result of the subexpression. Accepts namedparameters.Accepts any number of argument expressions.

Alphabetical Listing of Expression Operators

Name	Description
`$abs`	Returns the absolute value of a number.
`$acos`	Returns the inverse cosine (arc cosine) of a value in radians.
`$acosh`	Returns the inverse hyperbolic cosine (hyperbolic arc cosine) of a value in radians.
`$add`	Adds numbers to return the sum, or adds numbers and a date to returna new date. If adding numbers and a date, treats the numbers asmilliseconds. Accepts any number of argument expressions, but atmost, one expression can resolve to a date.
`$addToSet`	Returns an array of unique expression values for each group. Order of thearray elements is undefined.Available in `$group` stage only.
`$allElementsTrue`	Returns `true` if no element of a set evaluates to `false`,otherwise, returns `false`. Accepts a single argument expression.
`$and`	Returns `true` only when all its expressions evaluate to`true`. Accepts any number of argument expressions.
`$anyElementTrue`	Returns `true` if any elements of a set evaluate to `true`;otherwise, returns `false`. Accepts a single argument expression.
`$arrayElemAt`	Returns the element at the specified array index.
`$arrayToObject`	Converts an array of key value pairs to a document.
`$asin`	Returns the inverse sine (arc sine) of a value in radians.
`$asinh`	Returns the inverse hyperbolic sin (hyperbolic arc sine) of a value in radians.
`$atan`	Returns the inverse tangent (arc tangent) of a value in radians.
`$atan2`	Returns the inverse tangent (arc tangent) of `y / x` in radians,where `y` and `x` are the first and second values passed to theexpression respectively.
`$atanh`	Returns the inverse hyperbolic tangent (hyperbolic arc tangent) of a value in radians.
`$avg`	Returns an average of numerical values. Ignores non-numeric values.Changed in version 3.2: Available in both `$group` and `$project`stages.
`$ceil`	Returns the smallest integer greater than or equal to the specified number.
`$cmp`	Returns: `0` if the two values areequivalent, `1` if the first value is greater than thesecond, and `-1` if the first value is less than thesecond.
`$concat`	Concatenates any number of strings.
`$concatArrays`	Concatenates arrays to return the concatenated array.
`$cond`	A ternary operator that evaluates one expression, and depending onthe result, returns the value of one of the other two expressions.Accepts either three expressions in an ordered list or three namedparameters.
`$convert`	Converts a value to a specified type.
`$cos`	Returns the cosine of a value that is measured in radians.
`$dateFromParts`	Constructs a BSON Date object given the date’s constituentparts.
`$dateToParts`	Returns a document containing the constituent parts of a date.
`$dateFromString`	Returns a date/time as a date object.
`$dateToString`	Returns the date as a formatted string.
`$dayOfMonth`	Returns the day of the month for a date as a number between 1 and 31.
`$dayOfWeek`	Returns the day of the week for a date as a number between 1 (Sunday)and 7 (Saturday).
`$dayOfYear`	Returns the day of the year for a date as a number between 1 and 366(leap year).
`$degreesToRadians`	Converts a value from degrees to radians.
`$divide`	Returns the result of dividing the first number by the second. Acceptstwo argument expressions.
`$eq`	Returns `true` if the values areequivalent.
`$exp`	Raises e to the specified exponent.
`$filter`	Selects a subset of the array to return an array with only the elementsthat match the filter condition.
`$first`	Returns a value from the first document for each group. Order isonly defined if the documents are in a defined order.Available in `$group` stage only.
`$floor`	Returns the largest integer less than or equal to the specified number.
`$gt`	Returns `true` if the first value isgreater than the second.
`$gte`	Returns `true` if the first value isgreater than or equal to the second.
`$hour`	Returns the hour for a date as a number between 0 and 23.
`$ifNull`	Returns either the non-null result of the first expression or theresult of the second expression if the first expression results in anull result. Null result encompasses instances of undefined values ormissing fields. Accepts two expressions as arguments. The result ofthe second expression can be null.
`$in`	Returns a boolean indicating whether a specified value is in an array.
`$indexOfArray`	Searches an array for an occurence of a specified value and returnsthe array index of the first occurence. If the substring is not found,returns `-1`.
`$indexOfBytes`	Searches a string for an occurence of a substring and returns theUTF-8 byte index of the first occurence. If the substring is notfound, returns `-1`.
`$indexOfCP`	Searches a string for an occurence of a substring and returns theUTF-8 code point index of the first occurence. If thesubstring is not found, returns `-1`.
`$isArray`	Determines if the operand is an array. Returns a boolean.
`$isoDayOfWeek`	Returns the weekday number in ISO 8601 format, ranging from`1` (for Monday) to `7` (for Sunday).
`$isoWeek`	Returns the week number in ISO 8601 format, rangingfrom `1` to `53`. Weeknumbers start at `1` with the week (Monday through Sunday)that contains the year’s first Thursday.
`$isoWeekYear`	Returns the year number in ISO 8601 format. The year startswith the Monday of week 1 (ISO 8601) and ends with the Sunday of thelast week (ISO 8601).
`$last`	Returns a value from the last document for each group. Order isonly defined if the documents are in a defined order.Available in `$group` stage only.
`$let`	Defines variables for use within the scope of a subexpression andreturns the result of the subexpression. Accepts named parameters.Accepts any number of argument expressions.
`$literal`	Return a value without parsing. Use for values that the aggregationpipeline may interpret as an expression. For example, use a`$literal` expression to a string that starts with a`$` to avoid parsing as a field path.
`$ln`	Calculates the natural log of a number.
`$log`	Calculates the log of a number in the specified base.
`$log10`	Calculates the log base 10 of a number.
`$lt`	Returns `true` if the first value is lessthan the second.
`$lte`	Returns `true` if the first value is lessthan or equal to the second.
`$ltrim`	Removes whitespace or the specified characters from the beginning of a string.
`$map`	Applies a subexpression to each element of an array and returns thearray of resulting values in order. Accepts named parameters.
`$max`	Returns the highest expression value for each group.Changed in version 3.2: Available in both `$group` and `$project`stages.
`$mergeObjects`	Combines multiple documents into a single document.
`$meta`	Access text search metadata.
`$min`	Returns the lowest expression value for each group.Changed in version 3.2: Available in both `$group` and `$project`stages.
`$millisecond`	Returns the milliseconds of a date as a number between 0and 999.
`$minute`	Returns the minute for a date as a number between 0 and 59.
`$mod`	Returns the remainder of the first number divided by the second.Accepts two argument expressions.
`$month`	Returns the month for a date as a number between 1(January) and 12 (December).
`$multiply`	Multiplies numbers to return the product. Accepts any number ofargument expressions.
`$ne`	Returns `true` if the values are not_equivalent.
`$not`	Returns the boolean value that is the opposite of its argumentexpression. Accepts a single argument expression.
`$objectToArray`	Converts a document to an array of documents representing key-value pairs.
`$or`	Returns `true` when _any of its expressions evaluates to `true`.Accepts any number of argument expressions.
`$pow`	Raises a number to the specified exponent.
`$push`	Returns an array of expression values for each group.Available in `$group` stage only.
`$radiansToDegrees`	Converts a value from radians to degrees.
`$range`	Outputs an array containing a sequence of integers according touser-defined inputs.
`$reduce`	Applies an expression to each element in an array and combines theminto a single value.
`$regexFind`	Applies a regular expression (regex) to a string and returnsinformation on the first matched substring.
`$regexFindAll`	Applies a regular expression (regex) to a string and returnsinformation on the all matched substrings.
`$regexMatch`	Applies a regular expression (regex) to a string and returnsa boolean that indicates if a match is found or not.
`$reverseArray`	Returns an array with the elements in reverse order.
`$round`	Rounds a number to a whole integer or to a specified decimal place.
`$rtrim`	Removes whitespace or the specified characters from the end of a string.
`$second`	Returns the seconds for a date as a number between 0 and 60(leap seconds).
`$setDifference`	Returns a set with elements that appear in the first set but not inthe second set; i.e. performs arelative complement)of the second set relative to the first. Accepts exactly twoargument expressions.
`$setEquals`	Returns `true` if the input sets have the same distinct elements.Accepts two or more argument expressions.
`$setIntersection`	Returns a set with elements that appear in all of the input sets.Accepts any number of argument expressions.
`$setIsSubset`	Returns `true` if all elements of the first set appear in thesecond set, including when the first set equals the second set; i.e.not a strict subset.Accepts exactly two argument expressions.
`$setUnion`	Returns a set with elements that appear in any of the input sets.
`$size`	Returns the number of elements in the array. Accepts a singleexpression as argument.
`$sin`	Returns the sine of a value that is measured in radians.
`$slice`	Returns a subset of an array.
`$split`	Splits a string into substrings based on a delimiter. Returns anarray of substrings. If the delimiter is not found within the string,returns an array containing the original string.
`$sqrt`	Calculates the square root.
`$stdDevPop`	Returns the population standard deviation of the input values.Changed in version 3.2: Available in both `$group` and `$project`stages.
`$stdDevSamp`	Returns the sample standard deviation of the input values.Changed in version 3.2: Available in both `$group` and `$project`stages.
`$strcasecmp`	Performs case-insensitive string comparison and returns: `0` if twostrings are equivalent, `1` if the first string is greater than thesecond, and `-1` if the first string is less than the second.
`$strLenBytes`	Returns the number of UTF-8 encoded bytes in a string.
`$strLenCP`	Returns the number of UTF-8 code points in a string.
`$substr`	Deprecated. Use `$substrBytes` or `$substrCP`.
`$substrBytes`	Returns the substring of a string. Starts with thecharacter at the specified UTF-8 byte index (zero-based) in the stringand continues for the specified number of bytes.
`$substrCP`	Returns the substring of a string. Starts with thecharacter at the specified UTF-8 code point (CP) index (zero-based)in the string and continues for the number of code points specified.
`$subtract`	Returns the result of subtracting the second value from the first. Ifthe two values are numbers, return the difference. If the two valuesare dates, return the difference in milliseconds. If the two valuesare a date and a number in milliseconds, return the resulting date.Accepts two argument expressions. If the two values are a date and anumber, specify the date argument first as it is not meaningful tosubtract a date from a number.
`$sum`	Returns a sum of numerical values. Ignores non-numeric values.Changed in version 3.2: Available in both `$group` and `$project`stages.
`$switch`	Evaluates a series of case expressions. When it finds an expressionwhich evaluates to `true`, `$switch` executes a specifiedexpression and breaks out of the control flow.
`$tan`	Returns the tangent of a value that is measured in radians.
`$toBool`	Converts value to a boolean.
`$toDate`	Converts value to a Date.
`$toDecimal`	Converts value to a Decimal128.
`$toDouble`	Converts value to a double.
`$toInt`	Converts value to an integer.
`$toLong`	Converts value to a long.
`$toObjectId`	Converts value to an ObjectId.
`$toString`	Converts value to a string.
`$toLower`	Converts a string to lowercase. Accepts a single argument expression.
`$toUpper`	Converts a string to uppercase. Accepts a single argument expression.
`$trim`	Removes whitespace or the specified characters from the beginningand end of a string.
`$trunc`	Truncates a number to a whole integer or to a specified decimal place.
`$type`	Return the BSON data type of the field.
`$week`	Returns the week number for a date as a number between 0 (the partialweek that precedes the first Sunday of the year) and 53 (leap year).
`$year`	Returns the year for a date as a number (e.g. 2014).
`$zip`	Merge two arrays together.

For the pipeline stages, seeAggregation Pipeline Stages.