Proper Array sorting in javascript

Question

function isSortedAndHow(array) {
  let array_copy=array.slice(0,array.length);
  let ascending_arr=array.sort((a,b)=>{return a-b});
  let descending_arr=array.sort((a,b)=>{return b-a});
  
  //for checking array equality
  function element_check(arr1,arr2){
    return arr1.every((a1,a2)=>{a1===arr2[a2]})
  }
  if(element_check(array,ascending_arr)){
    return "yes,ascending order";
  }
  else if(element_check(array,descending_arr)){
      return "yes,descending order";
  }
  else{
    return "no";
  }
}

I'm trying to accept an array, check if it is sorted in any order and return output. However,the code is returning no all the time.

you have no return statement in arr1.every. btw, it is funny to use arrow functions and then a block with return instead of simple returning a value. — Nina Scholz
– Nina Scholz, Commented Jan 25, 2021 at 9:10
See When should I use a return statement in ES6 arrow functions. Also, you make array_copy but don't use it at all. ascending_arr, descending_arr, and array are all the same array. — VLAZ
– VLAZ, Commented Jan 25, 2021 at 9:16
@VLAZ Tht's an other issue with the code. If element_check was correct it would always return yes, ascending order since all 3 names refer to the same object. Given that it is incorrect it doesn't. — GACy20
– GACy20, Commented Jan 25, 2021 at 9:19

Nina Scholz · Accepted Answer · 2021-01-25 09:43:02Z

Because of the various errors

missing return statment in arrow function bis block statement,
using the same array for checking,
taking an superfluous sorting,

you could omit sorting and take a comparison function and iterate the given array directly with a short circuit function and comparison functions for wanted orders.

This approach takes inside of Array#every a check with the previous value or at index zero just true, because this item has no predecessor.

function isSortedAndHow(array) {
    const
        ascending = (a, b) => a <= b,
        descending = (a, b) => a >= b,
        check = (array, fn) => array.every((v, i, a) => !i || fn(a[i - 1], v));

    if (check(array, ascending)) return "yes, ascending order";
    if (check(array, descending)) return "yes, descending order";
    return "no";
}

console.log(isSortedAndHow([0, 1, 2, 5, 4]));
console.log(isSortedAndHow([0, 1, 2, 3, 4]));
console.log(isSortedAndHow([5, 4, 3, 2, 0]));

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