<!--info-header-start--><h1>Optional Keys <img src="https://img.shields.io/badge/-hard-de3d37" alt="hard"/> <img src="https://img.shields.io/badge/-%23utils-999" alt="#utils"/></h1><blockquote><p>by yituan <a href="https://github.com/yi-tuan" target="_blank">@yi-tuan</a></p></blockquote><p><a href="https://tsch.js.org/90/play" target="_blank"><img src="https://img.shields.io/badge/-Take%20the%20Challenge-3178c6?logo=typescript&logoColor=white" alt="Take the Challenge"/></a> &nbsp;&nbsp;&nbsp;<a href="./README.zh-CN.md" target="_blank"><img src="https://img.shields.io/badge/-%E7%AE%80%E4%BD%93%E4%B8%AD%E6%96%87-gray" alt="简体中文"/></a>  <a href="./README.ja.md" target="_blank"><img src="https://img.shields.io/badge/-%E6%97%A5%E6%9C%AC%E8%AA%9E-gray" alt="日本語"/></a> </p><!--info-header-end-->

Implement the advanced util type `OptionalKeys<T>`, which picks all the optional keys into a union.


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