4 open connections try to setVersion at the same time. 3 connections eventually close, allowing 1 setVersion call to proceed. On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS IndexedDB == null is false IndexedDB.open('set-version-queue') IndexedDB.open('set-version-queue') IndexedDB.open('set-version-queue') IndexedDB.open('set-version-queue') connections[0].setVersion('version 0') connections[1].setVersion('version 1') connections[2].setVersion('version 2') connections[3].setVersion('version 3') PASS event.version.length > 0 is true connection[1] received versionChangeEvent: version 0 PASS event.version.length > 0 is true connection[2] received versionChangeEvent: version 0 PASS event.version.length > 0 is true connection[3] received versionChangeEvent: version 0 PASS connection[0] got blocked event PASS event.version is "version 0" Close the connection that received the block event: connections[0].close() Close another connection as well, to test 4.7.4-note: connections[3].close() blocked0fired = true PASS event.version.length > 0 is true connection[2] received versionChangeEvent: version 1 PASS connection[1] got blocked event Ensure that this blocked event is in order: PASS blocked0fired is true PASS blocked2fired is false PASS event.version.length > 0 is true connection[1] received versionChangeEvent: version 2 PASS connection[2] got blocked event PASS event.version is "version 2" connections[2].close() blocked2fired = true PASS event.version.length > 0 is true connection[1] received versionChangeEvent: version 3 PASS connection[3] got blocked event Note: This means that a connection can receive a blocked event after its close() method has been called. Spec is silent on the issue and this is easiest to implement. PASS event.version is "version 3" PASS connection[0] got error event PASS event.target.errorCode is 13 PASS event.target.webkitErrorMessage.length > 0 is true Connection was closed before set version transaction was created PASS connection[2] got error event PASS event.target.errorCode is 13 PASS event.target.webkitErrorMessage.length > 0 is true Connection was closed before set version transaction was created PASS connection[3] got error event PASS event.target.errorCode is 13 PASS event.target.webkitErrorMessage.length > 0 is true Connection was closed before set version transaction was created PASS connection[1] got into SetVersion PASS successfullyParsed is true TEST COMPLETE