Hello. Thanks for your question.
You are right. We do invest considerable time of developers into testing. For front tier we do not have dedicated testing teams. Developers are expected to write and maintain tests for their code.
For node.js, we use some awesome open sourced modules on npm like jest, tape, mocha, jasmine(and many many more). Teams have their own freedom to explore on this front. For functional testing in browser, we have our own opensourced framework nemo.js (https://github.com/paypal/nemo). I've seen some teams use Phantom.js.
We do have full fledged Quality Assurance teams for testing Service infrastructures and full end-to-end testing of a product. We have specific environment setups for pre-build developer checks, Build check before release, and we have a Sandbox environment that mimics Production code to test without having to move money from our account and buying stuff online in the name of testing ;) :p