isSortedArray.js

Array.prototype.isSorted = function () {	
  if(this.length < 2) return false;  
  
  let dir = this[0] < this[1] ? 1 : -1;	  
  
  for(let i = 1; i < this.length-1; i++) {
    if(dir === 1 && this[i] > this[i+1]) {      
       return false;       
    }
    if(dir === -1 && this[i] < this[i+1]) {      
       return false;       
    }
  }
  return true;
}