fix: ingest WC as a simple object or number for w value

src/connection_string.ts

@@ -992,9 +992,16 @@ export const OPTIONS = {
             ...value
           }
         });
+      } else if (value === 'majority' || typeof value === 'number') {
+        return WriteConcern.fromOptions({
+          writeConcern: {
+            ...options.writeConcern,
+            w: value
+          }
+        });
       }
 
-      throw new MongoParseError(`WriteConcern must be an object, got ${JSON.stringify(value)}`);
+      throw new MongoParseError(`Invalid WriteConcern cannot parse: ${JSON.stringify(value)}`);
     }
   } as OptionDescriptor,
   wtimeout: {

src/mongo_client.ts

@@ -171,7 +171,7 @@ export interface MongoClientOptions extends BSONSerializeOptions, SupportedNodeC
   /** Allow a driver to force a Single topology type with a connection string containing one host */
   directConnection?: boolean;
 
-  /** The write concern */
+  /** The write concern w value */
   w?: W;
   /** The write concern timeout */
   wtimeoutMS?: number;

test/functional/write_concern.test.js

@@ -7,6 +7,9 @@ const generateTopologyTests = require('./spec-runner').generateTopologyTests;
 const loadSpecTests = require('../spec').loadSpecTests;
 const { withMonitoredClient } = require('./shared');
 
+const mock = require('../tools/mock');
+const { MongoClient } = require('../../src');
+
 describe('Write Concern', function () {
   describe('spec tests', function () {
     const testContext = new TestRunnerContext();
@@ -58,4 +61,40 @@ describe('Write Concern', function () {
       withMonitoredClient('insert', { queryOptions: { journal: true } }, journalOptionTest)
     );
   });
+
+  let server;
+  before(() => {
+    return mock.createServer().then(s => {
+      server = s;
+    });
+  });
+
+  after(() => {
+    mock.cleanup();
+  });
+
+  it('should pipe writeConcern from client down to API call', function () {
+    server.setMessageHandler(request => {
+      if (request.document && request.document.ismaster) {
+        return request.reply(mock.DEFAULT_ISMASTER);
+      }
+      expect(request.document.writeConcern).to.exist;
+      expect(request.document.writeConcern.w).to.equal('majority');
+      return request.reply({ ok: 1 });
+    });
+
+    const uri = `mongodb://${server.uri()}`;
+    const client = new MongoClient(uri, { writeConcern: 'majority' });
+    return client
+      .connect()
+      .then(() => {
+        const db = client.db('wc_test');
+        const collection = db.collection('wc');
+
+        return collection.insertMany([{ a: 2 }]);
+      })
+      .then(() => {
+        return client.close();
+      });
+  });
 });