example.js

const arrayDiffIndexes = (smallestArray, biggestArray) => {
  const common = {};
  
  smallestArray.forEach(item => {
    common[item.key] = true;
  });
  
  const diffIndexes = [];
  biggestArray.forEach((item, index) => {
    if (!common[item.key]) diffIndexes.push(index);
  });
	
  return diffIndexes; 	
}

const array1 = [{ key: 1 }, { key: 2 }, { key: 3 }];
const array2 = [{ key: 3 }, { key: 4 }, { key: 2 }, { key: 1 }];

const diffIndexes = arrayDiffIndexes(array1, array2);
console.log(diffIndexes);