<div class="type-placeholder" style="font-size: 15px; letter-spacing: -0.3px; color: #abb0b5; white-space: pre-wrap;" data-darkreader-inline-color="">
<p style="box-sizing: border-box; padding: 0px; line-height: 1.5; font-size: 16px; color: #ddd8d1; font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji'; letter-spacing: normal; white-space: normal; background-color: #151718; --darkreader-inline-color: #ded9d2; --darkreader-inline-bgcolor: #0c0d0f; margin: 0px !important 0px 12px 0px;" data-darkreader-inline-color="" data-darkreader-inline-bgcolor="">const parse Cookies = (cookie ='') =&gt;&nbsp;
<p style="box-sizing: border-box; margin: 0px 0px 12px; padding: 0px; line-height: 1.5; font-size: 16px; color: #ddd8d1; font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji'; letter-spacing: normal; white-space: normal; background-color: #151718; --darkreader-inline-color: #ded9d2; --darkreader-inline-bgcolor: #0c0d0f;" data-darkreader-inline-color="" data-darkreader-inline-bgcolor="">cookie.split(':')~~
<p style="box-sizing: border-box; margin: 0px 0px 12px; padding: 0px; line-height: 1.5; font-size: 16px; color: #ddd8d1; font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji'; letter-spacing: normal; white-space: normal; background-color: #151718; --darkreader-inline-color: #ded9d2; --darkreader-inline-bgcolor: #0c0d0f;" data-darkreader-inline-color="" data-darkreader-inline-bgcolor="">구문은 무슨 문법인가요 이해가잘안가서요&nbsp;
<p style="box-sizing: border-box; margin: 0px 0px 12px; padding: 0px; line-height: 1.5; font-size: 16px; color: #ddd8d1; font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji'; letter-spacing: normal; white-space: normal; background-color: #151718; --darkreader-inline-color: #ded9d2; --darkreader-inline-bgcolor: #0c0d0f;" data-darkreader-inline-color="" data-darkreader-inline-bgcolor="">const cookies = parseCookies(req.headers.cookie)
<p style="box-sizing: border-box; margin: 0px 0px 12px; padding: 0px; line-height: 1.5; font-size: 16px; color: #ddd8d1; font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji'; letter-spacing: normal; white-space: normal; background-color: #151718; --darkreader-inline-color: #ded9d2; --darkreader-inline-bgcolor: #0c0d0f;" data-darkreader-inline-color="" data-darkreader-inline-bgcolor="">cookies 는&nbsp;<span style="box-sizing: border-box; font-weight: inherit; font-style: inherit; color: #9cdcfe; font-family: Consolas, 'Courier New', monospace; font-size: 14px; white-space: pre; --darkreader-inline-color: #8ee4ff;" data-darkreader-inline-color="">req<span style="box-sizing: border-box; font-weight: inherit; font-style: inherit; background-color: #1e1e1e; color: #d4d4d4; font-family: Consolas, 'Courier New', monospace; font-size: 14px; white-space: pre; --darkreader-inline-bgcolor: #131415; --darkreader-inline-color: #dcd6d0;" data-darkreader-inline-bgcolor="" data-darkreader-inline-color="">.<span style="box-sizing: border-box; font-weight: inherit; font-style: inherit; color: #9cdcfe; font-family: Consolas, 'Courier New', monospace; font-size: 14px; white-space: pre; --darkreader-inline-color: #8ee4ff;" data-darkreader-inline-color="">headers<span style="box-sizing: border-box; font-weight: inherit; font-style: inherit; background-color: #1e1e1e; color: #d4d4d4; font-family: Consolas, 'Courier New', monospace; font-size: 14px; white-space: pre; --darkreader-inline-bgcolor: #131415; --darkreader-inline-color: #dcd6d0;" data-darkreader-inline-bgcolor="" data-darkreader-inline-color="">.<span style="box-sizing: border-box; font-weight: inherit; font-style: inherit; color: #9cdcfe; font-family: Consolas, 'Courier New', monospace; font-size: 14px; white-space: pre; --darkreader-inline-color: #8ee4ff;" data-darkreader-inline-color="">cookie 는 스트링인데 
<p style="box-sizing: border-box; margin: 0px 0px 12px; padding: 0px; line-height: 1.5; font-size: 16px; color: #ddd8d1; font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji'; letter-spacing: normal; white-space: normal; background-color: #151718; --darkreader-inline-color: #ded9d2; --darkreader-inline-bgcolor: #0c0d0f;" data-darkreader-inline-color="" data-darkreader-inline-bgcolor=""><span style="box-sizing: border-box; font-weight: inherit; font-style: inherit; color: #9cdcfe; font-family: Consolas, 'Courier New', monospace; font-size: 14px; white-space: pre; --darkreader-inline-color: #8ee4ff;" data-darkreader-inline-color=""><span style="box-sizing: border-box; font-weight: inherit; font-style: inherit; color: #f9f7f4; font-family: 'Spoqa Han Sans'; font-size: medium; white-space: normal; --darkreader-inline-color: #fbf2e6;" data-darkreader-inline-color="">&nbsp;(cookie ='') 이렇게생긴거에 어떻게 대입이 되는건가요?&nbsp;
</div>

default parameter 문법입니다.
parseCookies() 이런 식으로 아무것도 전달하지 않으면 cookie는자동으로 ''가 됩니다.

Inflearn Community Q&A

(cookie ='') 요부분