Script Considerations

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Constructors

Generator Functions
Array Sort
JavaScript Setters

The results of database queries cannot be passed inside the following contexts:

Class constructor functions
Non-async generator functions
Callbacks to .sort() on an array
JavaScript setters in classes

To access to the results of database calls, use async functions, async generator functions, or .map().

Constructors

The following constructors do not work:

// This code will fail
class FindResults {
   constructor() {
      this.value = db.students.find();
   }
}
// This code will fail
function listEntries() { return db.students.find(); }
class FindResults {
   constructor() {
      this.value = listEntries();
   }
}

Use an async function instead:

class FindResults {
   constructor() {
      this.value = ( async() => {
         return db.students.find();
      } )();
   }
}

Note

You can also create a method that performs a database operation inside a class as an alternative to working with asynchronous JavaScript.

class FindResults {
   constructor() { }
   init() { this.value = db.students.find(); }
 }

To use this class, first construct a class instance then call the .init() method.

Generator Functions

The following generator functions do not work:

// This code will fail
function* FindResults() {
   yield db.students.findOne();
}
// This code will fail
function listEntries() { return db.students.findOne(); }
function* findResults() {
   yield listEntries();
}

Use an async generator function instead:

function listEntries() { return db.students.findOne(); }
async function* findResults() {
   yield listEntries();
}

Array Sort

The following array sort does not work:

// This code will fail
db.getCollectionNames().sort( ( collectionOne, collectionTwo ) => {
  return db[ collectionOne ].estimatedDocumentCount() - db[ collectionOne ].estimatedDocumentCount() )
} );

Use .map() instead.

db.getCollectionNames().map( collectionName => {
   return { collectionName, size: db[ collectionName ].estimatedDocumentCount() };
} ).sort( ( collectionOne, collectionTwo ) => {
   return collectionOne.size - collectionTwo.size;
} ).map( collection => collection.collectionName);

This approach to array sort is often more performant than the equivalent unsupported code.

JavaScript Setters

The following JavaScript setter does not work:

// This code will fail
class TestClass {
  value = 1;
  get property() {
    return this.value;
  }
  // does not work:
  set property(value) {
    this.value = db.test.findOne({ value });
  }
}

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