[29athrowaway]

JavaScript runtimes are full of optimizations that execute conditionally on specific scenarios. If you don't follow the rewarding path of optimization you then go to the deeply unhappy path of deoptimization.

It's easy to get started with JavaScript, but to squeeze every bit of performance in JavaScript will quickly get you into a very complex topics that are not documented and change may change from version to version as they are often not "contracts".

The only way to learn those obscure topics is by reading the source code for your JavaScript implementation, their bug tracker, etc. This means if you have not been reading C++ or executed a non-JS profiler to make your JS faster then you probably have not gone far enough.

There are unexpected things that you would never have suspected if you had not read the implementation.