Strictly increasing or decreasing array - JavaScript

In Mathematics, a strictly increasing function is that function in which the value to be plotted always increase. Similarly, a strictly decreasing function is that function in which the value to be plotted always decrease.

We are required to write a JavaScript function that takes in an array of numbers and returns true if it’s either strictly increasing or strictly decreasing, otherwise returns false.

Example

Following is the code −

const arr = [12, 45, 6, 4, 23, 23, 21, 1];
const arr2 = [12, 45, 67, 89, 123, 144, 2656, 5657];
const sameSlope = (a, b, c) => (b - a < 0 && c - b < 0) || (b - a > 0 && c
- b > 0);
const increasingOrDecreasing = (arr = []) => {
   if(arr.length <= 2){
      return true;
   };
   for(let i = 1; i < arr.length - 1; i++){
      if(sameSlope(arr[i-1], arr[i], arr[i+1])){
         continue;
      };
      return false;
   };
   return true;
};
console.log(increasingOrDecreasing(arr));
console.log(increasingOrDecreasing(arr2));

Output

Following is the output in the console −

false
true