text: let outputScreen=document.getElementById("output-screen");
  
  function display(num) {
      outputScreen.value += num;
  }
  
  function calculate() {
     try{ outputScreen.value = eval(outputScreen.value);}
     catch (err) {
         alert("Invalid equation!")
     }
  }
  
  function allClear() {
      outputScreen.value = "";
  }
  
  function del() {
      outputScreen.value = outputScreen.value.slice(0, -1);
  }
  