Array of adjacent element's average - JavaScript

Let’s say, we have an array of numbers −

const arr = [3, 5, 7, 8, 3, 5, 7, 4, 2, 8, 4, 2, 1];

We are required to write a function that returns an array with the average of the corresponding element and its predecessor. For the first element, as there are no predecessors, so that very element should be returned.

Let’s write the code for this function, we will use the Array.prototype.map() function to solve this problem −

Example

const arr = [3, 5, 7, 8, 3, 5, 7, 4, 2, 8, 4, 2, 1];
const consecutiveAverage = arr => {
   return arr.map((el, ind, array) => {
      const first = (array[ind-1] || 0);
      const second = (1 + !!ind);
      return ((el + first) / second);
   });
};
console.log(consecutiveAverage(arr));

Output

This will produce the following output in console −

[
   3,   4, 6, 7.5, 5.5, 4,
   6, 5.5, 3,   5,   6, 3,
   1.5
]