diff --git a/src/collection.ts b/src/collection.ts
index 8fcdf90bff..3665498995 100644
--- a/src/collection.ts
+++ b/src/collection.ts
@@ -106,6 +106,8 @@ export interface CollectionOptions extends BSONSerializeOptions, WriteConcernOpt
   readConcern?: ReadConcernLike;
   /** The preferred read preference (ReadPreference.PRIMARY, ReadPreference.PRIMARY_PREFERRED, ReadPreference.SECONDARY, ReadPreference.SECONDARY_PREFERRED, ReadPreference.NEAREST). */
   readPreference?: ReadPreferenceLike;
+  /** @internal TODO(NODE-5688): make this public */
+  timeoutMS?: number;
 }
 
 /** @internal */
diff --git a/src/cursor/abstract_cursor.ts b/src/cursor/abstract_cursor.ts
index 51cf4ce700..10aa5eea5f 100644
--- a/src/cursor/abstract_cursor.ts
+++ b/src/cursor/abstract_cursor.ts
@@ -109,6 +109,8 @@ export interface AbstractCursorOptions extends BSONSerializeOptions {
    */
   awaitData?: boolean;
   noCursorTimeout?: boolean;
+  /** @internal TODO(NODE-5688): make this public */
+  timeoutMS?: number;
 }
 
 /** @internal */
@@ -184,6 +186,7 @@ export abstract class AbstractCursor<
           : ReadPreference.primary,
       ...pluckBSONSerializeOptions(options)
     };
+    this[kOptions].timeoutMS = options.timeoutMS;
 
     const readConcern = ReadConcern.fromOptions(options);
     if (readConcern) {
diff --git a/src/db.ts b/src/db.ts
index 1dda4834c2..423979a0d4 100644
--- a/src/db.ts
+++ b/src/db.ts
@@ -66,7 +66,8 @@ const DB_OPTIONS_ALLOW_LIST = [
   'enableUtf8Validation',
   'promoteValues',
   'compression',
-  'retryWrites'
+  'retryWrites',
+  'timeoutMS'
 ];
 
 /** @internal */
@@ -94,6 +95,8 @@ export interface DbOptions extends BSONSerializeOptions, WriteConcernOptions {
   readConcern?: ReadConcern;
   /** Should retry failed writes */
   retryWrites?: boolean;
+  /** @internal TODO(NODE-5688): make this public */
+  timeoutMS?: number;
 }
 
 /**
diff --git a/src/gridfs/download.ts b/src/gridfs/download.ts
index 08659dfabf..6b0fc57fc6 100644
--- a/src/gridfs/download.ts
+++ b/src/gridfs/download.ts
@@ -28,6 +28,8 @@ export interface GridFSBucketReadStreamOptions {
    * to be returned by the stream. `end` is non-inclusive
    */
   end?: number;
+  /** @internal TODO(NODE-5688): make this public */
+  timeoutMS?: number;
 }
 
 /** @public */
diff --git a/src/gridfs/index.ts b/src/gridfs/index.ts
index c7611c852b..51c32b7a01 100644
--- a/src/gridfs/index.ts
+++ b/src/gridfs/index.ts
@@ -36,6 +36,8 @@ export interface GridFSBucketOptions extends WriteConcernOptions {
   chunkSizeBytes?: number;
   /** Read preference to be passed to read operations */
   readPreference?: ReadPreference;
+  /** @internal TODO(NODE-5688): make this public */
+  timeoutMS?: number;
 }
 
 /** @internal */
diff --git a/src/gridfs/upload.ts b/src/gridfs/upload.ts
index 80024d7d32..12440f2670 100644
--- a/src/gridfs/upload.ts
+++ b/src/gridfs/upload.ts
@@ -36,6 +36,8 @@ export interface GridFSBucketWriteStreamOptions extends WriteConcernOptions {
    * @deprecated Will be removed in the next major version. Add an aliases field to the metadata document instead.
    */
   aliases?: string[];
+  /** @internal TODO(NODE-5688): make this public */
+  timeoutMS?: number;
 }
 
 /**
diff --git a/src/mongo_client.ts b/src/mongo_client.ts
index 98a9f151ac..1e21aefe35 100644
--- a/src/mongo_client.ts
+++ b/src/mongo_client.ts
@@ -120,7 +120,7 @@ export type SupportedNodeConnectionOptions = SupportedTLSConnectionOptions &
 export interface MongoClientOptions extends BSONSerializeOptions, SupportedNodeConnectionOptions {
   /** Specifies the name of the replica set, if the mongod is a member of a replica set. */
   replicaSet?: string;
-  /** @internal This option is in development and currently has no behaviour. */
+  /** @internal TODO(NODE-5688): This option is in development and currently has no behaviour.  */
   timeoutMS?: number;
   /** Enables or disables TLS/SSL for the connection. */
   tls?: boolean;
@@ -897,4 +897,6 @@ export interface MongoOptions
    * TODO: NODE-5671 - remove internal flag
    */
   mongodbLogPath?: 'stderr' | 'stdout' | MongoDBLogWritable;
+  /** @internal TODO(NODE-5688): make this public */
+  timeoutMS?: number;
 }
diff --git a/src/operations/execute_operation.ts b/src/operations/execute_operation.ts
index 59f58f8748..b2951170ad 100644
--- a/src/operations/execute_operation.ts
+++ b/src/operations/execute_operation.ts
@@ -110,6 +110,11 @@ export async function executeOperation<
   } else if (session.client !== client) {
     throw new MongoInvalidArgumentError('ClientSession must be from the same MongoClient');
   }
+  if (session.explicit && session?.timeoutMS != null && operation.options.timeoutMS != null) {
+    throw new MongoInvalidArgumentError(
+      'Do not specify timeoutMS on operation if already specified on an explicit session'
+    );
+  }
 
   const readPreference = operation.readPreference ?? ReadPreference.primary;
   const inTransaction = !!session?.inTransaction();
diff --git a/src/operations/operation.ts b/src/operations/operation.ts
index 7f564cbc27..e71baa44a9 100644
--- a/src/operations/operation.ts
+++ b/src/operations/operation.ts
@@ -34,6 +34,9 @@ export interface OperationOptions extends BSONSerializeOptions {
   /** @internal Hints to `executeOperation` that this operation should not unpin on an ended transaction */
   bypassPinningCheck?: boolean;
   omitReadPreference?: boolean;
+
+  /** @internal TODO(NODE-5688): make this public */
+  timeoutMS?: number;
 }
 
 /** @internal */
diff --git a/src/sessions.ts b/src/sessions.ts
index 4822f0483a..7ef4c3ba22 100644
--- a/src/sessions.ts
+++ b/src/sessions.ts
@@ -61,6 +61,9 @@ export interface ClientSessionOptions {
   snapshot?: boolean;
   /** The default TransactionOptions to use for transactions started on this session. */
   defaultTransactionOptions?: TransactionOptions;
+  /** @internal
+   * The value of timeoutMS used for CSOT. Used to override client timeoutMS */
+  defaultTimeoutMS?: number;
 
   /** @internal */
   owner?: symbol | AbstractCursor;
@@ -131,6 +134,8 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
   [kPinnedConnection]?: Connection;
   /** @internal */
   [kTxnNumberIncrement]: number;
+  /** @internal */
+  timeoutMS?: number;
 
   /**
    * Create a client session.
@@ -173,6 +178,7 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
     this.sessionPool = sessionPool;
     this.hasEnded = false;
     this.clientOptions = clientOptions;
+    this.timeoutMS = options.defaultTimeoutMS ?? client.options?.timeoutMS;
 
     this.explicit = !!options.explicit;
     this[kServerSession] = this.explicit ? this.sessionPool.acquire() : null;
diff --git a/test/integration/client-side-operations-timeout/node_csot.test.ts b/test/integration/client-side-operations-timeout/node_csot.test.ts
index 06e8179daf..b6a936afbb 100644
--- a/test/integration/client-side-operations-timeout/node_csot.test.ts
+++ b/test/integration/client-side-operations-timeout/node_csot.test.ts
@@ -1,4 +1,97 @@
-/* eslint-disable @typescript-eslint/no-empty-function */
 /* Anything javascript specific relating to timeouts */
+import { expect } from 'chai';
+import * as sinon from 'sinon';
 
-describe.skip('CSOT driver tests', () => {});
+import {
+  type ClientSession,
+  type Collection,
+  type Db,
+  type FindCursor,
+  type MongoClient
+} from '../../mongodb';
+
+describe('CSOT driver tests', () => {
+  afterEach(() => {
+    sinon.restore();
+  });
+
+  describe('timeoutMS inheritance', () => {
+    let client: MongoClient;
+    let db: Db;
+    let coll: Collection;
+
+    beforeEach(async function () {
+      client = this.configuration.newClient(undefined, { timeoutMS: 100 });
+      db = client.db('test', { timeoutMS: 200 });
+    });
+
+    afterEach(async () => {
+      await client?.close();
+    });
+
+    describe('when timeoutMS is provided on an operation', () => {
+      beforeEach(() => {
+        coll = db.collection('test', { timeoutMS: 300 });
+      });
+
+      describe('when in a session', () => {
+        let cursor: FindCursor;
+        let session: ClientSession;
+
+        beforeEach(() => {
+          session = client.startSession({ defaultTimeoutMS: 400 });
+          cursor = coll.find({}, { session, timeoutMS: 500 });
+        });
+
+        afterEach(async () => {
+          await cursor?.close();
+          await session?.endSession();
+          await session.endSession();
+        });
+
+        it('throws an error', async () => {
+          expect(cursor.cursorOptions).to.have.property('timeoutMS', 500);
+        });
+      });
+
+      describe('when not in a session', () => {
+        let cursor: FindCursor;
+
+        beforeEach(() => {
+          db = client.db('test', { timeoutMS: 200 });
+          coll = db.collection('test', { timeoutMS: 300 });
+          cursor = coll.find({}, { timeoutMS: 400 });
+        });
+
+        afterEach(async () => {
+          await cursor?.close();
+        });
+
+        it('overrides the value provided on the db', async () => {
+          expect(cursor.cursorOptions).to.have.property('timeoutMS', 400);
+        });
+      });
+    });
+
+    describe('when timeoutMS is provided on a collection', () => {
+      beforeEach(() => {
+        db = client.db('test', { timeoutMS: 200 });
+        coll = db.collection('test', { timeoutMS: 300 });
+      });
+
+      it('overrides the value provided on the db', () => {
+        expect(coll.s.options).to.have.property('timeoutMS', 300);
+      });
+
+      describe('when timeoutMS is provided on a db', () => {
+        beforeEach(() => {
+          db = client.db('test', { timeoutMS: 200 });
+        });
+
+        it('overrides the value provided on the client', () => {
+          expect(db.s.options).to.have.property('timeoutMS', 200);
+        });
+      });
+    });
+  });
+});
diff --git a/test/spec/uri-options/connection-options.json b/test/spec/uri-options/connection-options.json
index 8bb05cc721..b2669b6cf1 100644
--- a/test/spec/uri-options/connection-options.json
+++ b/test/spec/uri-options/connection-options.json
@@ -2,7 +2,7 @@
   "tests": [
     {
       "description": "Valid connection and timeout options are parsed correctly",
-      "uri": "mongodb://example.com/?appname=URI-OPTIONS-SPEC-TEST&connectTimeoutMS=20000&heartbeatFrequencyMS=5000&localThresholdMS=3000&maxIdleTimeMS=50000&replicaSet=uri-options-spec&retryWrites=true&serverSelectionTimeoutMS=15000&socketTimeoutMS=7500",
+      "uri": "mongodb://example.com/?appname=URI-OPTIONS-SPEC-TEST&connectTimeoutMS=20000&heartbeatFrequencyMS=5000&localThresholdMS=3000&maxIdleTimeMS=50000&replicaSet=uri-options-spec&retryWrites=true&serverSelectionTimeoutMS=15000&socketTimeoutMS=7500&timeoutMS=100",
       "valid": true,
       "warning": false,
       "hosts": null,
@@ -16,7 +16,8 @@
         "replicaSet": "uri-options-spec",
         "retryWrites": true,
         "serverSelectionTimeoutMS": 15000,
-        "socketTimeoutMS": 7500
+        "socketTimeoutMS": 7500,
+        "timeoutMS": 100
       }
     },
     {
@@ -238,6 +239,35 @@
       "hosts": null,
       "auth": null,
       "options": {}
+    },
+    {
+      "description": "timeoutMS=0",
+      "uri": "mongodb://example.com/?timeoutMS=0",
+      "valid": true,
+      "warning": false,
+      "hosts": null,
+      "auth": null,
+      "options": {
+        "timeoutMS": 0
+      }
+    },
+    {
+      "description": "Non-numeric timeoutMS causes a warning",
+      "uri": "mongodb://example.com/?timeoutMS=invalid",
+      "valid": true,
+      "warning": true,
+      "hosts": null,
+      "auth": null,
+      "options": {}
+    },
+    {
+      "description": "Too low timeoutMS causes a warning",
+      "uri": "mongodb://example.com/?timeoutMS=-2",
+      "valid": true,
+      "warning": true,
+      "hosts": null,
+      "auth": null,
+      "options": {}
     }
   ]
 }
diff --git a/test/spec/uri-options/connection-options.yml b/test/spec/uri-options/connection-options.yml
index 83f63daa1a..43ef9ec9b3 100644
--- a/test/spec/uri-options/connection-options.yml
+++ b/test/spec/uri-options/connection-options.yml
@@ -1,7 +1,7 @@
 tests:
     -
         description: "Valid connection and timeout options are parsed correctly"
-        uri: "mongodb://example.com/?appname=URI-OPTIONS-SPEC-TEST&connectTimeoutMS=20000&heartbeatFrequencyMS=5000&localThresholdMS=3000&maxIdleTimeMS=50000&replicaSet=uri-options-spec&retryWrites=true&serverSelectionTimeoutMS=15000&socketTimeoutMS=7500"
+        uri: "mongodb://example.com/?appname=URI-OPTIONS-SPEC-TEST&connectTimeoutMS=20000&heartbeatFrequencyMS=5000&localThresholdMS=3000&maxIdleTimeMS=50000&replicaSet=uri-options-spec&retryWrites=true&serverSelectionTimeoutMS=15000&socketTimeoutMS=7500&timeoutMS=100"
         valid: true
         warning: false
         hosts: ~
@@ -16,6 +16,7 @@ tests:
             retryWrites: true
             serverSelectionTimeoutMS: 15000
             socketTimeoutMS: 7500
+            timeoutMS: 100
     -
         description: "Non-numeric connectTimeoutMS causes a warning"
         uri: "mongodb://example.com/?connectTimeoutMS=invalid"
@@ -64,7 +65,7 @@ tests:
         hosts: ~
         auth: ~
         options: {}
-    -
+    - 
         description: "Invalid retryWrites causes a warning"
         uri: "mongodb://example.com/?retryWrites=invalid"
         valid: true
@@ -207,3 +208,28 @@ tests:
       hosts: ~
       auth: ~
       options: {}
+    -
+        description: "timeoutMS=0"
+        uri: "mongodb://example.com/?timeoutMS=0"
+        valid: true
+        warning: false
+        hosts: ~
+        auth: ~
+        options:
+            timeoutMS: 0
+    -
+        description: "Non-numeric timeoutMS causes a warning"
+        uri: "mongodb://example.com/?timeoutMS=invalid"
+        valid: true
+        warning: true
+        hosts: ~
+        auth: ~
+        options: {}
+    -
+        description: "Too low timeoutMS causes a warning"
+        uri: "mongodb://example.com/?timeoutMS=-2"
+        valid: true
+        warning: true
+        hosts: ~
+        auth: ~
+        options: {}
diff --git a/test/spec/uri-options/connection-pool-options.yml b/test/spec/uri-options/connection-pool-options.yml
index 65fbb7367c..0333d4e31e 100644
--- a/test/spec/uri-options/connection-pool-options.yml
+++ b/test/spec/uri-options/connection-pool-options.yml
@@ -65,4 +65,3 @@ tests:
       hosts: ~
       auth: ~
       options: {}
-      
\ No newline at end of file
diff --git a/test/spec/uri-options/read-preference-options.json b/test/spec/uri-options/read-preference-options.json
index df8c0c0eb8..cdac6a63c3 100644
--- a/test/spec/uri-options/read-preference-options.json
+++ b/test/spec/uri-options/read-preference-options.json
@@ -23,7 +23,7 @@
     },
     {
       "description": "Single readPreferenceTags is parsed as array of size one",
-      "uri": "mongodb://example.com/?readPreferenceTags=dc:ny",
+      "uri": "mongodb://example.com/?readPreference=secondary&readPreferenceTags=dc:ny",
       "valid": true,
       "warning": false,
       "hosts": null,
diff --git a/test/spec/uri-options/read-preference-options.yml b/test/spec/uri-options/read-preference-options.yml
index cac8c6d89c..4ced669acf 100644
--- a/test/spec/uri-options/read-preference-options.yml
+++ b/test/spec/uri-options/read-preference-options.yml
@@ -17,7 +17,7 @@ tests:
             maxStalenessSeconds: 120
     -
         description: "Single readPreferenceTags is parsed as array of size one"
-        uri: "mongodb://example.com/?readPreferenceTags=dc:ny"
+        uri: "mongodb://example.com/?readPreference=secondary&readPreferenceTags=dc:ny"
         valid: true
         warning: false
         hosts: ~
diff --git a/test/tools/uri_spec_runner.ts b/test/tools/uri_spec_runner.ts
index 6ad6e86381..844e5bd470 100644
--- a/test/tools/uri_spec_runner.ts
+++ b/test/tools/uri_spec_runner.ts
@@ -349,6 +349,7 @@ export function executeUriValidationTest(
       case 'maxConnecting':
       case 'maxPoolSize':
       case 'minPoolSize':
+      case 'timeoutMS':
       case 'connectTimeoutMS':
       case 'heartbeatFrequencyMS':
       case 'localThresholdMS':
diff --git a/test/unit/mongo_client.test.js b/test/unit/mongo_client.test.ts
similarity index 97%
rename from test/unit/mongo_client.test.js
rename to test/unit/mongo_client.test.ts
index e3b3beb92b..79663a2603 100644
--- a/test/unit/mongo_client.test.js
+++ b/test/unit/mongo_client.test.ts
@@ -1,28 +1,28 @@
-('use strict');
-
+import { expect } from 'chai';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as sinon from 'sinon';
+import { Writable } from 'stream';
 import { inspect } from 'util';
 
-const os = require('os');
-const fs = require('fs');
-const { expect } = require('chai');
-const { getSymbolFrom } = require('../tools/utils');
-const { parseOptions, resolveSRVRecord } = require('../mongodb');
-const { ReadConcern } = require('../mongodb');
-const { WriteConcern } = require('../mongodb');
-const { ReadPreference } = require('../mongodb');
-const { MongoCredentials } = require('../mongodb');
-const {
+import {
+  MongoAPIError,
   MongoClient,
+  type MongoClientOptions,
+  MongoCredentials,
+  MongoInvalidArgumentError,
+  MongoLoggableComponent,
+  MongoLogger,
   MongoParseError,
+  parseOptions,
+  ReadConcern,
+  ReadPreference,
+  resolveSRVRecord,
   ServerApiVersion,
-  MongoAPIError,
-  MongoInvalidArgumentError
-} = require('../mongodb');
-const { MongoLogger } = require('../mongodb');
-// eslint-disable-next-line no-restricted-modules
-const { SeverityLevel, MongoLoggableComponent } = require('../../src/mongo_logger');
-const sinon = require('sinon');
-const { Writable } = require('stream');
+  SeverityLevel,
+  WriteConcern
+} from '../mongodb';
+import { getSymbolFrom } from '../tools/utils';
 
 describe('MongoClient', function () {
   it('MongoClient should always freeze public options', function () {
@@ -144,7 +144,10 @@ describe('MongoClient', function () {
   };
 
   it('should parse all options from the options object', function () {
-    const options = parseOptions('mongodb://localhost:27017/', ALL_OPTIONS);
+    const options = parseOptions(
+      'mongodb://localhost:27017/',
+      ALL_OPTIONS as unknown as MongoClientOptions
+    );
     // Check consolidated options
     expect(options).has.property('writeConcern');
     expect(options.writeConcern).has.property('w', 2);
@@ -1205,4 +1208,28 @@ describe('MongoClient', function () {
       expect.fail('missed exception');
     });
   });
+
+  context('timeoutMS', function () {
+    context('when timeoutMS is negative', function () {
+      it('throws MongoParseError', function () {
+        expect(() => new MongoClient('mongodb://host', { timeoutMS: -1 })).to.throw(
+          MongoParseError
+        );
+      });
+    });
+
+    context('when timeoutMS is positive', function () {
+      it('sets the value on the MongoClient', function () {
+        const client = new MongoClient('mongodb://host', { timeoutMS: 1 });
+        expect(client.options.timeoutMS).to.equal(1);
+      });
+    });
+
+    context('when timeoutMS is zero', function () {
+      it('sets the value on the MongoClient', function () {
+        const client = new MongoClient('mongodb://host', { timeoutMS: 0 });
+        expect(client.options.timeoutMS).to.equal(0);
+      });
+    });
+  });
 });
diff --git a/test/unit/sessions.test.js b/test/unit/sessions.test.ts
similarity index 90%
rename from test/unit/sessions.test.js
rename to test/unit/sessions.test.ts
index ba58fef41e..f889ed9c6a 100644
--- a/test/unit/sessions.test.js
+++ b/test/unit/sessions.test.ts
@@ -1,21 +1,21 @@
-'use strict';
+import { expect } from 'chai';
+import * as sinon from 'sinon';
 
-const mock = require('../tools/mongodb-mock/index');
-const { expect } = require('chai');
-const { genClusterTime } = require('../tools/common');
-const {
-  ServerSessionPool,
-  ServerSession,
-  ClientSession,
+import {
   applySession,
-  BSON
-} = require('../mongodb');
-const { now, isHello } = require('../mongodb');
-const { getSymbolFrom } = require('../tools/utils');
-const { Long } = require('../mongodb');
-const { MongoRuntimeError } = require('../mongodb');
-const sinon = require('sinon');
-const { MongoClient } = require('../mongodb');
+  BSON,
+  ClientSession,
+  isHello,
+  Long,
+  MongoClient,
+  MongoRuntimeError,
+  now,
+  ServerSession,
+  ServerSessionPool
+} from '../mongodb';
+import { genClusterTime } from '../tools/common';
+import * as mock from '../tools/mongodb-mock/index';
+import { getSymbolFrom } from '../tools/utils';
 
 describe('Sessions - unit', function () {
   let client;
@@ -272,6 +272,60 @@ describe('Sessions - unit', function () {
         const txnNumberIncrementSymbol = getSymbolFrom(session, 'txnNumberIncrement');
         expect(session).to.have.property(txnNumberIncrementSymbol, 0);
       });
+
+      describe('defaultTimeoutMS', function () {
+        let client: MongoClient;
+        let session: ClientSession;
+        let server;
+
+        beforeEach(async () => {
+          server = await mock.createServer();
+        });
+
+        afterEach(async () => {
+          await mock.cleanup();
+        });
+
+        context('when client has defined timeoutMS', function () {
+          beforeEach(async () => {
+            client = new MongoClient(`mongodb://${server.hostAddress()}`, { timeoutMS: 100 });
+          });
+
+          context('when defaultTimeoutMS is defined', function () {
+            it(`overrides client's timeoutMS value`, function () {
+              session = new ClientSession(client, serverSessionPool, { defaultTimeoutMS: 200 });
+              expect(session).to.have.property('timeoutMS', 200);
+            });
+          });
+
+          context('when defaultTimeoutMS is not defined', function () {
+            it(`inherits client's timeoutMS value`, function () {
+              session = new ClientSession(client, serverSessionPool, {});
+              expect(session).to.have.property('timeoutMS', 100);
+            });
+          });
+        });
+
+        context('when client has not defined timeoutMS', function () {
+          beforeEach(async () => {
+            client = new MongoClient(`mongodb://${server.hostAddress()}`, {});
+          });
+
+          context('when defaultTimeoutMS is defined', function () {
+            it(`sets timeoutMS to defaultTimeoutMS`, function () {
+              session = new ClientSession(client, serverSessionPool, { defaultTimeoutMS: 200 });
+              expect(session).to.have.property('timeoutMS', 200);
+            });
+          });
+
+          context('when defaultTimeoutMS is not defined', function () {
+            it(`leaves timeoutMS as undefined`, function () {
+              session = new ClientSession(client, serverSessionPool, {});
+              expect(session.timeoutMS).to.be.undefined;
+            });
+          });
+        });
+      });
     });
 
     describe('get serverSession()', () => {
@@ -442,7 +496,7 @@ describe('Sessions - unit', function () {
     beforeEach(async () => {
       server = await mock.createServer();
       server.setMessageHandler(request => {
-        var doc = request.document;
+        const doc = request.document;
         if (isHello(doc)) {
           request.reply(Object.assign({}, mock.HELLO, { logicalSessionTimeoutMinutes: 10 }));
         }