One implementation could be:

function groupBy(collection, property) {
  const output = {};

  if (!collection || typeof collection !== 'object') {
    return output;
  }

  const isPropertyFunction = typeof property === 'function';
  const isPropertyPath = typeof property === 'string';

  for (const value of Object.values(collection)) {
    let current = undefined;

    if (isPropertyFunction) {
      current = property(value);
    } else if (isPropertyPath) {
      // a.b.c -&gt; [a, b, c];
      const path = property.split('.');
      let i;
      let currentKey;
      let currentItem = value; // { a: { b: { c: 1 } } }

      for (i = 0; i &lt; path.length; i++) {
        // [a, b, c] -&gt; currentKey: path[0] -&gt; a
        // [a, b, c] -&gt; currentKey: path[1] -&gt; b
        // [a, b, c] -&gt; currentKey: path[2] -&gt; c
        currentKey = path[i];

        if (!currentItem[currentKey]) {
          currentItem = undefined;
          break;
        }

        // { a: { b: { c: 1 } } } -&gt; a??
        // currentItem = { b: { c: 1 } }

        // { b: { c: 1 } } -&gt; b??
        // currentItem = { c: 1 }

        // { c: 1 } -&gt; c??
        // currentItem = 1
        currentItem = currentItem[currentKey];
      }

      current = currentItem;
    }

    output[current] = output[current] || [];
    output[current].push(value);
  }

  return output;
}

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How to implement lodash groupBy functionality from scratch? | JavaScript Interview Question | Frontend Problem Solving

@Yomesh Gupta