MongoDB $add Operator

MongoDB provides different types of arithmetic expression operators that are used in the aggregation pipeline stages and $add operator is one of them. This operator is used to add numbers or dates. If $add operator adds date, then it will treat other arguments as milliseconds and add to the specified date.

Syntax: 

{ $add: [ <Expression1>, <Expression2>, ... <ExpressionN>] }

Here, the Expression must be a valid expression like numbers or a date.

Examples:

In the following examples, we are working with:

Database: GeeksforGeeks

Collection: Employee

Document: four documents that contain the details of the employees in the form of field-value pairs.

Adding numbers using $add operator:

In this example, we are going to find the total salary of every employee in the development department. 

db.Employee.aggregate([{$match: {department: "Development"}},
... {$project: {name: 1, tSalary: {$add: 
                ["$firstSalary", "$secondSalary"]}}}])

Adding numbers in the embedded document using $add operator:

In this example, we are going to find a total of three months’ salary of the employee in the HR department. 

db.Employee.aggregate([{$match: {department: "HR"}},
... {$project: {name: 1, tSalary: {$add: ["$salary.firstMonth", 
                                          "$salary.secondMonth",
                                          "$salary.thirdMonth"]}}}])

Adding date using $add operator: 

In this example, we are going to extend the last date of the project by adding 5 days (i.e, 5*24*60*60000)  in the projectEndDate field of the testing department. 

db.Employee.aggregate([{$match: {department: "Testing"}},
... {$project: {extendprojectDate: 
          {$add: ["$projectEndDate", 5*24*60*60000]}}}])

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