Cleanup

From review of PR #56 (watch2 support), I have a feeling that we should be able to clean up both the source code and all of the tests. I am raising this for future consideration and discussion.

Starting to add a list:

- [ ] use const & let instead of var, especially in new code (noticed after merging PR #56)
- [ ] look for ways to make the code easier to read & understand, especially the code added in PRs #56 & #76

/cc @l3ender

先审查 PR #56 和 PR #76，然后检查受其更改影响的源代码和测试。将宽泛的清理请求拆分为具体的可读性和 var-to-const/let 任务，每项任务都要明确其代码和测试，并定义预期的完成状态。

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