diff --git a/.github/ISSUE_TEMPLATE/bug.yml b/.github/ISSUE_TEMPLATE/bug.yml
index 3089b69e90e..804597275f7 100644
--- a/.github/ISSUE_TEMPLATE/bug.yml
+++ b/.github/ISSUE_TEMPLATE/bug.yml
@@ -37,6 +37,14 @@ body:
     validations:
       required: true
 
+  - type: input
+    id: typescript-version
+    attributes:
+      label: Typescript version (if applicable)
+      placeholder: 4.8.x
+    validations:
+      required: false
+
   - type: textarea
     id: description
     attributes:
diff --git a/docs/guide.md b/docs/guide.md
index 54651a6ed35..454dac270cf 100644
--- a/docs/guide.md
+++ b/docs/guide.md
@@ -744,9 +744,9 @@ The read option also allows us to specify _tag sets_. These tell the
 [driver](https://github.com/mongodb/node-mongodb-native/) from which members
 of the replica-set it should attempt to read. Read more about tag sets
 [here](http://docs.mongodb.org/manual/applications/replication/#tag-sets) and
-[here](http://mongodb.github.com/node-mongodb-native/driver-articles/anintroductionto1_1and2_2.html#read-preferences).
+[here](https://www.mongodb.com/docs/manual/core/read-preference).
 
-_NOTE: you may also specify the driver read pref [strategy](http://mongodb.github.com/node-mongodb-native/api-generated/replset.html?highlight=strategy)
+_NOTE: you may also specify the driver read preference [strategy](https://www.mongodb.com/docs/manual/core/read-preference/#read-preference-modes)
 option when connecting:_
 
 ```javascript
diff --git a/lib/aggregate.js b/lib/aggregate.js
index 2426a256ca1..7178cce7526 100644
--- a/lib/aggregate.js
+++ b/lib/aggregate.js
@@ -111,11 +111,11 @@ Aggregate.prototype.model = function(model) {
   this._model = model;
   if (model.schema != null) {
     if (this.options.readPreference == null &&
-        model.schema.options.read != null) {
+      model.schema.options.read != null) {
       this.options.readPreference = model.schema.options.read;
     }
     if (this.options.collation == null &&
-        model.schema.options.collation != null) {
+      model.schema.options.collation != null) {
       this.options.collation = model.schema.options.collation;
     }
   }
@@ -158,7 +158,7 @@ Aggregate.prototype.append = function() {
  * Requires MongoDB v3.4+ to work
  *
  * #### Example:
-  *
+ *
  *     // adding new fields based on existing fields
  *     aggregate.addFields({
  *         newField: '$b.nested'
@@ -328,6 +328,28 @@ Aggregate.prototype.project = function(arg) {
  * @api public
  */
 
+/**
+ * Appends a new $fill operator to this aggregate pipeline.
+ *
+ * #### Example:
+ *
+ *      aggregate.fill({
+ *        output: {
+ *          bootsSold: { value: 0 },
+ *          sandalsSold: { value: 0 },
+ *          sneakersSold: { value: 0 }
+ *        }
+ *      });
+ *
+ * @see $fill https://www.mongodb.com/docs/manual/reference/operator/aggregation/fill/
+ * @method fill
+ * @memberOf Aggregate
+ * @instance
+ * @param {Object} arg $fill operator contents
+ * @return {Aggregate}
+ * @api public
+ */
+
 /**
  * Appends a new $geoNear operator to this aggregate pipeline.
  *
@@ -366,7 +388,7 @@ Aggregate.prototype.near = function(arg) {
  * define methods
  */
 
-'group match skip limit out densify'.split(' ').forEach(function($operator) {
+'group match skip limit out densify fill'.split(' ').forEach(function($operator) {
   Aggregate.prototype[$operator] = function(arg) {
     const op = {};
     op['$' + $operator] = arg;
@@ -702,7 +724,7 @@ Aggregate.prototype.readConcern = function(level) {
 Aggregate.prototype.redact = function(expression, thenExpr, elseExpr) {
   if (arguments.length === 3) {
     if ((typeof thenExpr === 'string' && !validRedactStringValues.has(thenExpr)) ||
-        (typeof elseExpr === 'string' && !validRedactStringValues.has(elseExpr))) {
+      (typeof elseExpr === 'string' && !validRedactStringValues.has(elseExpr))) {
       throw new Error('If thenExpr or elseExpr is string, it must be either $$DESCEND, $$PRUNE or $$KEEP');
     }
 
@@ -1099,9 +1121,7 @@ Aggregate.prototype.catch = function(reject) {
 
 if (Symbol.asyncIterator != null) {
   Aggregate.prototype[Symbol.asyncIterator] = function() {
-    return this.cursor({ useMongooseAggCursor: true }).
-      transformNull().
-      _transformForAsyncIterator();
+    return this.cursor({ useMongooseAggCursor: true }).transformNull()._transformForAsyncIterator();
   };
 }
 
diff --git a/lib/connection.js b/lib/connection.js
index ccf14825d18..98fb871c115 100644
--- a/lib/connection.js
+++ b/lib/connection.js
@@ -1534,6 +1534,17 @@ Connection.prototype.syncIndexes = async function syncIndexes(options = {}) {
  *
  * Returns a new connection object, with the new db.
  *
+ * #### Example:
+ *
+ *     // Connect to `initialdb` first
+ *     const conn = await mongoose.createConnection('mongodb://localhost:27017/initialdb').asPromise();
+ *
+ *     // Creates an un-cached connection to `mydb`
+ *     const db = conn.useDb('mydb');
+ *     // Creates a cached connection to `mydb2`. All calls to `conn.useDb('mydb2', { useCache: true })` will return the same
+ *     // connection instance as opposed to creating a new connection instance
+ *     const db2 = conn.useDb('mydb2', { useCache: true });
+ *
  * @method useDb
  * @memberOf Connection
  * @param {String} name The database name
diff --git a/test/aggregate.test.js b/test/aggregate.test.js
index 168db7a7e34..91acbf8716e 100644
--- a/test/aggregate.test.js
+++ b/test/aggregate.test.js
@@ -389,6 +389,25 @@ describe('aggregate: ', function() {
     });
   });
 
+  describe('fill', function() {
+    it('works', function() {
+      const aggregate = new Aggregate();
+      const obj = {
+        output:
+          {
+            bootsSold: { value: 0 },
+            sandalsSold: { value: 0 },
+            sneakersSold: { value: 0 }
+          }
+      };
+
+      aggregate.fill(obj);
+
+      assert.equal(aggregate._pipeline.length, 1);
+      assert.deepEqual(aggregate._pipeline[0].$fill, obj);
+    });
+  });
+
   describe('model()', function() {
     it('works', function() {
       const aggregate = new Aggregate();
diff --git a/test/types/PipelineStage.test.ts b/test/types/PipelineStage.test.ts
index 29c4696caca..28670945721 100644
--- a/test/types/PipelineStage.test.ts
+++ b/test/types/PipelineStage.test.ts
@@ -311,6 +311,85 @@ const setWindowFields4: PipelineStage = {
   }
 };
 
+const setWindowFieldsLinearFill: PipelineStage = {
+  $setWindowFields: {
+    partitionBy: '$stock',
+    sortBy: { date: 1 },
+    output: {
+      price: { $linearFill: '$price' }
+    }
+  }
+};
+
+const setWindowFieldsLocf: PipelineStage = {
+  $setWindowFields: {
+    partitionBy: '$stock',
+    sortBy: { date: 1 },
+    output: {
+      price: { $locf: '$price' }
+    }
+  }
+};
+
+const fillWithOutput: PipelineStage = {
+  $fill: {
+    output: {
+      bootsSold: { value: 0 }
+    }
+  }
+};
+
+const fillWithPartitionBy: PipelineStage = {
+  $fill: {
+    partitionBy: 'date',
+    output: {
+      bootsSold: { value: 0 }
+    }
+  }
+};
+
+const fillWithPartitionByFields: PipelineStage = {
+  $fill: {
+    partitionByFields: ['date'],
+    output: {
+      bootsSold: { value: 0 }
+    }
+  }
+};
+
+const fillWithSortBy: PipelineStage = {
+  $fill: {
+    sortBy: {
+      date: -1
+    },
+    output: {
+      bootsSold: { value: 0 }
+    }
+  }
+};
+
+const fillWithOutputMethodLinear: PipelineStage = {
+  $fill: {
+    sortBy: {
+      date: -1
+    },
+    output: {
+      bootsSold: { method: 'linear' }
+    }
+  }
+};
+
+const fillWithOutputMethodLocf: PipelineStage = {
+  $fill: {
+    sortBy: {
+      date: -1
+    },
+    output: {
+      bootsSold: { method: 'locf' }
+    }
+  }
+};
+
 const group1: PipelineStage = { $group: { _id: null, ageStdDev: { $stdDevSamp: '$age' } } };
 const group2: PipelineStage = {
   $group: {
diff --git a/types/aggregate.d.ts b/types/aggregate.d.ts
index df5b4efdcc6..5352e22d0f6 100644
--- a/types/aggregate.d.ts
+++ b/types/aggregate.d.ts
@@ -133,6 +133,9 @@ declare module 'mongoose' {
     /** Combines multiple aggregation pipelines. */
     facet(options: PipelineStage.Facet['$facet']): this;
 
+    /** Appends a new $fill operator to this aggregate pipeline */
+    fill(arg: PipelineStage.Fill['$fill']): this;
+
     /** Appends new custom $graphLookup operator(s) to this aggregate pipeline, performing a recursive search on a collection. */
     graphLookup(options: PipelineStage.GraphLookup['$graphLookup']): this;
 
diff --git a/types/expressions.d.ts b/types/expressions.d.ts
index c2d83ed87dd..cf48bef8538 100644
--- a/types/expressions.d.ts
+++ b/types/expressions.d.ts
@@ -1180,6 +1180,26 @@ declare module 'mongoose' {
       $last: Expression;
     }
 
+    export interface LinearFill {
+      /**
+       * Fills null and missing fields in a window using linear interpolation based on surrounding field values.
+       *
+       * @version 5.3
+       * @see https://www.mongodb.com/docs/manual/reference/operator/aggregation/linearFill
+       */
+      $linearFill: Expression
+    }
+
+    export interface Locf {
+      /**
+       * Last observation carried forward. Sets values for null and missing fields in a window to the last non-null value for the field.
+       *
+       * @version 5.2
+       * @see https://www.mongodb.com/docs/manual/reference/operator/aggregation/locf
+       */
+      $locf: Expression
+    }
+
     export interface Map {
       /**
        * Applies a subexpression to each element of an array and returns the array of resulting values in order. Accepts named parameters.
@@ -2735,6 +2755,8 @@ declare module 'mongoose' {
     Expression.First |
     Expression.Integral |
     Expression.Last |
+    Expression.LinearFill |
+    Expression.Locf |
     Expression.Max |
     Expression.Min |
     Expression.Push |
diff --git a/types/pipelinestage.d.ts b/types/pipelinestage.d.ts
index 9086a4afb30..f58e4b61f27 100644
--- a/types/pipelinestage.d.ts
+++ b/types/pipelinestage.d.ts
@@ -8,7 +8,9 @@ declare module 'mongoose' {
     | PipelineStage.BucketAuto
     | PipelineStage.CollStats
     | PipelineStage.Count
+    | PipelineStage.Densify
     | PipelineStage.Facet
+    | PipelineStage.Fill
     | PipelineStage.GeoNear
     | PipelineStage.GraphLookup
     | PipelineStage.Group
@@ -89,6 +91,16 @@ declare module 'mongoose' {
       }
     }
 
+    export interface Fill {
+      /** [`$fill` reference](https://docs.mongodb.com/manual/reference/operator/aggregation/fill/) */
+      $fill: {
+        partitionBy?: Expression,
+        partitionByFields?: string[],
+        sortBy?: Record<string, 1 | -1>,
+        output: Record<string, { value: Expression } | { method: 'linear' | 'locf' }>
+      }
+    }
+
     export interface Facet {
       /** [`$facet` reference](https://docs.mongodb.com/manual/reference/operator/aggregation/facet/) */
       $facet: Record<string, FacetPipelineStage[]>;
@@ -201,7 +213,7 @@ declare module 'mongoose' {
 
     export interface ReplaceWith {
       /** [`$replaceWith` reference](https://docs.mongodb.com/manual/reference/operator/aggregation/replaceWith/) */
-      $replaceWith: ObjectExpressionOperator;
+      $replaceWith: ObjectExpressionOperator | { [field: string]: Expression };
     }
 
     export interface Sample {