for (var i = 1, target = document.getElementById('f' + i + 'x'); target; document.getElementById('fxx').value += target.value, i++, target = document.getElementById('f' + i + 'x'));
便于阅读:
var i = 1;
var target = document.getElementById('f' + i + 'x');
for (; target; ) {
document.getElementById('fxx').value += target.value;
i++;
target = document.getElementById('f' + i + 'x');
}