let arr = [1,2,3,4,5];
/// with initial value
let result = arr.reduce((accumulator,currentValue,currentIndex,array)=>{
  console.log(`result: ${accumulator}, current value: ${currentValue}, index: ${currentIndex}, array: ${array} `);
  return accumulator + currentValue;
},0);
///OUTUT:
result: 0, current value: 1, index: 0, array: 1,2,3,4,5
result: 1, current value: 2, index: 1, array: 1,2,3,4,5
result: 3, current value: 3, index: 2, array: 1,2,3,4,5
result: 6, current value: 4, index: 3, array: 1,2,3,4,5
result: 10, current value: 5, index: 4, array: 1,2,3,4,5


/// without initial value
let result = arr.reduce((accumulator,currentValue,currentIndex,array)=>{
  console.log(`result: ${accumulator}, current value: ${currentValue}, index: ${currentIndex}, array: ${array} `);
  return accumulator + currentValue;
});
/// OUTPUT:
result: 1, current value: 2, index: 1, array: 1,2,3,4,5
result: 3, current value: 3, index: 2, array: 1,2,3,4,5
result: 6, current value: 4, index: 3, array: 1,2,3,4,5
result: 10, current value: 5, index: 4, array: 1,2,3,4,5