This array reassignment does not make sense to me

So Im trying to solve this algorithm where I have to sum 4 numbers from given array(first parameter) in order to get second passed parameter, and Im doing it in a pretty stupid way (question not about solving the algorithm). Question is: why I can't delete values from array and recreate/reassign itself again, hope that make sence. Is it just how Javascript works or I did something wrong? Thanks in advance!

function foo(arr1, sum){
      let arr = arr1;
      for(let i=0; i<9999;i++){
        let val1=arr[Math.floor(Math.random()*arr.length)];
        arr.splice(arr.indexOf(val1),1);
         console.log(arr1);
        let val2=arr[Math.floor(Math.random()*arr.length)];
        arr.splice(arr.indexOf(val2),1);
        let val3=arr[Math.floor(Math.random()*arr.length)];
        arr.splice(arr.indexOf(val3),1);
        let val4=arr[Math.floor(Math.random()*arr.length)];
        arr.splice(arr.indexOf(val4),1);
        if(val1+val2+val3+val4 == sum){
          console.log(val1,val2,val3,val4);
         return [val1,val2,val3,val4]; 
        }

       arr=arr1;
      }
    }
    console.log(foo([2, 7, 4, 0, 9, 5, 1, 3], 20));