I have two arrays:

a = [12, 50, 2, 5, 6];

and

b = [0, 1, 3];

I want to sum those arrays value in array A with exact index value as array B so that would be 12+50+5 = 67. Kindly help me to do this in native javascript. I already tried searching but I can't find any luck. I found related article below, but I can't get the logic

indexOf method in an object array?

I have two arrays:

a = [12, 50, 2, 5, 6];

and

b = [0, 1, 3];

I want to sum those arrays value in array A with exact index value as array B so that would be 12+50+5 = 67. Kindly help me to do this in native javascript. I already tried searching but I can't find any luck. I found related article below, but I can't get the logic

indexOf method in an object array?

• javascript
• arrays

• 1 index 3 value is not 5? – passion Commented Sep 30, 2016 at 1:07

5 Answers 5

You can simply do as follows;

var arr = [12, 50, 2, 5, 6],
    idx = [0, 1, 3],
    sum = idx.map(i => arr[i])
             .reduce((p,c) => p + c);
console.log(sum);

sumAIndiciesOfB = function (a, b) {
    var runningSum = 0;
    for(var i = 0; b.length; i++) {
        runningSum += a[b[i]];
    }
    return runningSum;
};

logic explained:

loop through array b. For each value in b, look it up in array a (a[b[i]]) and then add it to runningSum. After looping through b you will have summed each index of a and the total will be in runningSum.

b contains the indices of a to sum, so loop over b, referencing a:

var sum=0, i;
for (i=0;i<b.length;i++)
{
  sum = sum + a[b[i]];
}
// sum now equals your result

You could simply reduce array a and only add values if their index exists in array b.

a.reduce((prev, curr, index) => b.indexOf(index) >= 0 ? prev+curr : prev, 0)

The result is 12+50+5=67.

Like this:

function addByIndexes(numberArray, indexArray){
  var n = 0;
  for(var i=0,l=indexArray.length; i<l; i++){
    n += numberArray[indexArray[i]];
  }
  return n;
}
console.log(addByIndexes([12, 50, 2, 5, 6], [0, 1, 3]));