style: simplify code

[LinLin00000000]

Checklist

• [x] I have ensured my pull request is not behind the main or master branch of the original repository.
• [x] I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• [x] I have written a commit message that passes commitlint linting.
• [x] I have ensured that my code changes pass linting tests.
• [x] I have ensured that my code changes pass unit tests.
• [x] I have described my pull request and the reasons for code changes along with context if necessary.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (bff06e9) 100.00% compared to head (d3bee7f) 100.00%.

:exclamation: Current head d3bee7f differs from pull request most recent head e70e9ea. Consider uploading reports for the commit e70e9ea to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #172 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 17 16 -1 Branches 5 4 -1 ========================================= - Hits 17 16 -1 ``` | [Impacted Files](https://codecov.io/gh/koajs/compose/pull/172?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=koajs) | Coverage Δ | | |---|---|---| | [index.js](https://codecov.io/gh/koajs/compose/pull/172?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=koajs#diff-aW5kZXguanM=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=koajs). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=koajs)

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