fix: merge sessionId instead of removing it (#119)

It is unexpected for "uppdateContext" to remove
the automatically generated sessionId. This fix keeps the
sessionId as long as it is not specified in the new
context passed in to the "updateContext" method.

fixes: #118

src/index.test.ts

@@ -818,6 +818,36 @@ test('Should update context fields on request', async () => {
     expect(url.searchParams.get('environment')).toEqual('prod');
 });
 
+test('Should not replace sessionId when updating context', async () => {
+    fetchMock.mockResponses(
+        [JSON.stringify(data), { status: 200 }],
+        [JSON.stringify(data), { status: 304 }]
+    );
+    const config: IConfig = {
+        url: 'http://localhost/test',
+        clientKey: '12',
+        appName: 'web',
+        environment: 'prod',
+    };
+    const client = new UnleashClient(config);
+    await client.start();
+    const context = client.getContext();
+    await client.updateContext({
+        userId: '123',
+        remoteAddress: 'address',
+        properties: {
+            property1: 'property1',
+            property2: 'property2',
+        },
+    });
+
+    jest.advanceTimersByTime(1001);
+
+    const url = new URL(getTypeSafeRequestUrl(fetchMock));
+
+    expect(url.searchParams.get('sessionId')).toEqual(context.sessionId?.toString());
+});
+
 test('Should not add property fields when properties is an empty object', async () => {
     fetchMock.mockResponses(
         [JSON.stringify(data), { status: 200 }],

src/index.ts

@@ -230,6 +230,7 @@ export class UnleashClient extends TinyEmitter {
         const staticContext = {
             environment: this.context.environment,
             appName: this.context.appName,
+            sessionId: this.context.sessionId,
         };
         this.context = { ...staticContext, ...context };
         if (this.timerRef) {