Test Driven Development (TDD) is a great mindset for building new components & services. The Angular / React / Vue CLIs make this super easy with ready-made spec.ts / test.tsx / test.ts files. For the SAS side, tools such as Karma let you execute services (a)synchronously, using different user identities and input data. You can then perform assertions on the outputs to test particular workflows, security settings, and results. Tests are incorporated into the build / deploy process, and the results support the readiness of production release.
Browser compatibility is another area to consider. Do you need to support IE? What about Chrome, Firefox? Tools such as Browserstack make it easy to try out your app on different devices, browsers, and versions.
Performance testing tools such as Jmeter let you to test the concurrency of your app, and scalability of your environment. Do you have enough Multibridge connections? Are your SAS services performant? Does your database handle simultaneous reads/writes?
Usability / Security testing should also be performed as early as possible to ensure the product is fit for purpose, and safe.
Testing is a huuuge topic - there is plenty more to consider. What did I miss?