var bmi = weight / (height * height);
if (bmi <= 18.5) {
     alert('light');
} else if ( 18.5 < bmi <= 25) {
     alert('normal');
} else if (25 < bmi <= 28) {
     alert('weight');
} else if (28 < bmi <= 32) {
     alert('fat');
} else {
     alert('very fat');
}