Edit in JSFiddle

function acTion(){
    var number = 10;
    
    //증가연산자 (선행)
    document.getElementById('output1').innerHTML = ++number + '<br>'; 
    //증가연산자(후행)
    document.getElementById('output4').innerHTML = number-- + '<br>';
    document.getElementById('output5').innerHTML = number + '<br><br>';
    
    number = 20;
    
    //감소연산자(선행)
    document.getElementById('output6').innerHTML = --number + '<br>';
    
    //감소연산자(후행)
    document.getElementById('output7').innerHTML = number-- + '<br>';
    document.getElementById('output8').innerHTML = number;
}