One possible implementation could be:

function customAt(index) {
  if (index &gt;= 0) {
    return this[index];
  } else {
    return this[this.length + index];
  }
}

Array.prototype.customAt = customAt;

Other Frontend Problems:

https://devtools.tech/resources/s/how-to-improve-your-codebase-or-custom-eslint-rules-or-advanced-javascript-or-devtools-tech---rid---KgSipm8RngCFwwtI7bh2

https://devtools.tech/resources/s/implementation-of-reduce-polyfill-or-devtools-tech---rid---RPwJsAj15EZKfZsMT4V7

https://devtools.tech/resources/s/how-to-improve-performance-of-your-website-or-cumulative-layout-shift-or-part-1---rid---iFdJmQ4rTsUUv72yhH59

https://devtools.tech/resources/s/build-your-own-redux-or-part-1-or-advanced-frontend---rid---MNkFAzvLnmds66wD3JV0

https://devtools.tech/resources/s/create-your-own-redux-or-part-2-or-createstore-api-or-advanced-frontend---rid---H9EEn90tE3Jag6zCfNkM

https://devtools.tech/resources/s/what-every-react-developer-should-know-or-part-1-or-stale-closures---rid---x3PZ2ib2HWoab1RdaT2H

https://devtools.tech/resources/s/things-every-engineer-should-know-about-promises-in-javascript-or-frontend-fundamentals---rid---CpzShsPEajyOwTavUu5O

https://devtools.tech/resources/s/javascript-promises-fundamentals-every-engineer-should-know-or-part-2-or-event-loop-or-microtasks---rid---oBrzK6Mt7HNAnKBT3ogC

https://devtools.tech/questions/all

How to implement Array.prototype.at? JavaScript Interview Question | Problem Solving | JavaScript Polyfills

@Yomesh Gupta