Reduceのサンプル. 収集するaccumulatorと初期値を設定するのが重要

reduce.ts

const array1 = [1, 2, 3, 4,5];

const reducer = (accumulator, currentValue,index) => {
  console.log(`---`)
  console.log(`Index: ${index}`)
  console.log(`A: ${accumulator}`)
  console.log(`C: ${currentValue}`)
  return {value: accumulator.value + currentValue}
  return accumulator+currentValue
}

// 1 + 2 + 3 + 4
console.log(JSON.stringify(array1.reduce(reducer,{value:0})));
// expected output: 10