async run forEach and get results

forEachAsync.js

const forEachAsync = async (arr, handler, isSerial = true) => {
  if( isSerial ) {
    const result = [];

    return arr.reduce((acc, val) => {
      return acc.then(()=>handler(val))
        .then(ret=>{
          result.push(ret);
          return ret;
        });
    }, Promise.resolve())
    .then(()=>{
      return Promise.resolve(result);
    });
  }
  else {
    return Promise.all(arr.map(handler))
  }
}

// test - case1
forEachAsync([1,2,3], (item)=>{
  console.log(item);
})

// test - case2
(
  async ()=>{
    console.log(
      await forEachAsync([1,2,3], async (item)=>{
        return new Promise((resolve, reject)=>{
          setTimeout(()=>{
            // console.log(item);
            resolve(item);
          }, Math.random() * 100 )
        })
      }, false)
    )
  }
)()