SOURCE

const arr = [1, 6, 43, 3, 56, 0, 9, 98];

function composeNewArray(arr, l) {
  const newArr = [];
  for (let i=0;i<l;i++) {
    while(1) {
      let index = Math.floor(Math.random() * (arr.length - 1));
			if (newArr.indexOf(arr[index])!== -1) {
        continue;
      } else {
      	newArr.push(arr[index]);
        break;
      }
    }    
  }
  return newArr;
}

const newArr = composeNewArray(arr, 4);
console.log(newArr);