MongoDB query to set user defined variable into query?

For user-defined variables, use var keyword in MongoDB. Let us create a collection with documents −

> db.demo327.insertOne({"FirstName":"Chris","LastName":"Brown"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e516952f8647eb59e562076") } > db.demo327.insertOne({"FirstName":"David","LastName":"Miller"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e51695af8647eb59e562077") } > db.demo327.insertOne({"FirstName":"John","LastName":"Doe"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e516962f8647eb59e562078") } > db.demo327.insertOne({"FirstName":"John","LastName":"Smith"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e516968f8647eb59e562079") }

Display all documents from a collection with the help of find() method −

> db.demo327.find();

This will produce the following output −

{ "_id" : ObjectId("5e516952f8647eb59e562076"), "FirstName" : "Chris", "LastName" : "Brown" } { "_id" : ObjectId("5e51695af8647eb59e562077"), "FirstName" : "David", "LastName" : "Miller" } { "_id" : ObjectId("5e516962f8647eb59e562078"), "FirstName" : "John", "LastName" : "Doe" } { "_id" : ObjectId("5e516968f8647eb59e562079"), "FirstName" : "John", "LastName" : "Smith" }

Following is how to set a user-defined variable into the query −

> var name="John"; > db.demo327.find({"FirstName":name});

This will produce the following output −

{ "_id" : ObjectId("5e516962f8647eb59e562078"), "FirstName" : "John", "LastName" : "Doe" } { "_id" : ObjectId("5e516968f8647eb59e562079"), "FirstName" : "John", "LastName" : "Smith" }