One possible implementation could be

function customFindIndex(callback) {
  // DO NOT REMOVE
  'use strict';

  // Write your solution below
  if (this === null || this === undefined) {
    throw new TypeError('Array.prototype.customFindIndex called on null or undefined');
  } else if (typeof callback !== 'function') {
    throw new TypeError('callback must be a function');
  }

  var list = Object(this);
  var thisArg = arguments[1];
  var currentValue = undefined;
  var length = list.length || 0;

  // write your solution below
  for (let i = 0; i &lt; length; i++) {
    currentValue = list[i];

    if (callback.call(thisArg, currentValue, i, list)) {
      return i;
    }
  }

  return -1;
}

Array.prototype.customFindIndex = customFindIndex;

Other Frontend Problems:

https://devtools.tech/questions/s/how-to-implement-arrayprototypefind-javascript-interview-question-or-problem-solving-or-javascript-polyfills---qid---nX72m7o44hxRUlSnbjo5

https://devtools.tech/questions/s/how-to-implement-arrayprototypeat-javascript-interview-question-or-problem-solving-or-javascript-polyfills---qid---cbVcZ8jIXITU5wD6hIrb

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.findIndex? JavaScript Interview Question | Problem Solving | JavaScript Polyfills

@Yomesh Gupta