fix: more consistent implementation

Author: dariakp

src/sessions.ts

@@ -169,10 +169,10 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
     this[kServerSession] = this.explicit ? this.sessionPool.acquire() : null;
     this[kTxnNumberIncrement] = 0;
 
-    const canEnableCausalConsistency = this.explicit && options.snapshot !== true;
+    const defaultCausalConsistencyValue = this.explicit && options.snapshot !== true;
     this.supports = {
       // if we can enable causal consistency, do so by default
-      causalConsistency: canEnableCausalConsistency && options.causalConsistency !== false
+      causalConsistency: options.causalConsistency ?? defaultCausalConsistencyValue
     };
 
     this.clusterTime = options.initialClusterTime;

test/unit/sessions.test.js

@@ -193,18 +193,20 @@ describe('Sessions - unit', function () {
         expect(session.supports).property('causalConsistency', false);
       });
 
-      it('should set `causalConsistency` to `false` in implicit sessions regardless of options', function () {
-        const sessionTrue = new ClientSession(client, serverSessionPool, {
+      it('should respect `causalConsistency=false` option in implicit sessions', function () {
+        const session = new ClientSession(client, serverSessionPool, {
           explicit: false,
-          causalConsistency: true
+          causalConsistency: false
         });
-        expect(sessionTrue.supports).property('causalConsistency', false);
+        expect(session.supports).property('causalConsistency', false);
+      });
 
-        const sessionFalse = new ClientSession(client, serverSessionPool, {
+      it('should respect `causalConsistency=true` option in implicit sessions', function () {
+        const session = new ClientSession(client, serverSessionPool, {
           explicit: false,
-          causalConsistency: false
+          causalConsistency: true
         });
-        expect(sessionFalse.supports).property('causalConsistency', false);
+        expect(session.supports).property('causalConsistency', true);
       });
 
       it('should default to `null` for `clusterTime`', function () {