This code is much easier to understand as it do not add levels of indentation and follows the principle where the 0 indentation level is the principal path of the application where other paths are exceptions or rare cases.

Only test the happy path, but make sure to add a test case for any regression that couldn’t have been caught at lower levels with better tests (for example, if a regression is found, regression tests should be added at the lowest level possible).

The “happy path” or “success track” is the straight path from start to the terminus named success.

The error callback becomes a reject, while the "happy path" callback becomes a resolve.