如何在具有最新日期的数组中查找 object 的索引

Question

I have an array of objects in an array. Each object has a date field. Here is a method I wrote to retrieve the index of the object with the newest date, works fine:

GetIndexOfLatestDate()
   {
    var indexOfLatestDate:number = 0;
    
    var maxDate:number = new Date(this.objArray[0].date).getTime();
    
    for(var nIndex:number = 1; nIndex < this.m_objArray.length; nIndex++)
    {
      if(new Date(this.objArray[nIndex].date).getTime() > maxDate)
      {
        maxDate = new Date(this.objArray[nIndex].date).getTime();
        indexOFLatestDate = nIndex;
      }
    }

    return indexOfLatestDate;
   }

How can this be written (much) more succinctly?

Thanks for any help.

Answer 1

You can do it with a reduce , something like:

index = this.objArray.reduce((accum, value, index) => {
   if(!accum){
      accum = {
          index,
          maxDate: value.date
      };
   } else {
      if(accum.maxDate.getTime() > value.date.getTime()){
          accum = {
              index,
              maxDate: value.date
          };
      }
   }

   return accum;
}

}, null).index;

Answer 2

You can do this using built-in function like this

 const array1 = [{date: '2/5/2021'}, {date: '3/11/2019'}, {date: '12/9/2022'}]; const dateArray = array1.map(({date}) => {return new Date(date)}) const maxDate = Math.max(...dateArray); const indexMaxElem = dateArray.findIndex(dateObj => dateObj.getTime() === maxDate) console.log(indexMaxElem)

It is less efficient though, since it needs to do multiple pass through the array

Answer 3

let dateArr = [];
objArray.forEach(item => {

    //  extract the dates from the source array to form new array
    dateArr.push(objArray.date.getTime();
});

// find the maximum date in this array, which will have the same index
indexOfLatest = dateArr.findIndex(Math.max(...dateArr));

Answer 4

GetIndexOfLatestDate(objArray){ let max = objArray.reduce(function (a, b){ return new Date(a.date) > new Date(b.date) ? a : b; }); return objArray.indexOf(max); }

Answer 5

I would suggest using the reduce function that javascript provides. This solution also doesn't loop through the array multiple times, and it calls new Date().getTime() once per date.

GetIndexOfLatestDate()
{
   if (this.objectArr === null || this.objectArr.length === 0) {
      return null;
   }

   return this.objectArr.reduce((accum, value, index) => {
      const newDate = new Date(value.date).getTime();
      return newDate > accum.maxDate ? {index, maxDate: newDate} : accum;
   }, {index: 0, maxDate: new Date(this.objectArr[0].date).getTime()}).index;
}

---

if this looks too confusing, here is an expanded version that's easier to follow if you are new to the reduce function.

GetIndexOfLatestDate()
{
   // check if object arr is empty
   if (this.objectArr === null || this.objectArr.length === 0) {
      return null;
   }

   // set default accumulator for first passthrough
   const defaultAccum = {  
         index: 0, 
         maxDate: new Date(this.objectArr[0].date).getTime()
    }

   const maxValueWithIndex = this.objectArr.reduce((accum, value, index) => {
      // set formatted date to prevent multiple Date() calls
      const newDate = new Date(value.date).getTime();

      // if the new date is larger than the current largest date, set
      // the accumulator to the new largest date and its index
      if (newDate > accum.maxDate)
         accum = {
            index: index, 
            maxDate: newDate
         };
      } 
      
      // return the current accumulator, i.e. the current largest date
      return accum;
    }, defaultAccum);

   // return the index of the latest date
   return maxValueWithIndex.index;
}