内衣尺码测量页面代码优化

闲暇之余给公司做了个内衣尺码测量页面，其中涉及到内衣尺码各阶段判断，由于不涉及到数据库，所以代码直接用javascript来写，现贴上JS代码供 友友们讨论，看是否能优化下代码，让执行效率更高点。 

<script type="text/javascript" language="javascript">

    function braSize_onClick(){                                        //内衣尺码测量函数

        var numUp = document.getElementById("numUp").value;

        var numDown = document.getElementById("numDown").value;

        var result = numUp - numDown;

        if(numDown != "" && numUp != ""){

            if(numDown < 68 || numDown > 92){alert("对不起，您的下围尺码不在测量范围内，请重输。");}

            else if(numDown < 73){document.getElementById("braSize").value = 70}

            else if(numDown < 78){document.getElementById("braSize").value = 75}

            else if(numDown < 83){document.getElementById("braSize").value = 80}

            else if(numDown < 88){document.getElementById("braSize").value = 85}

            else if(numDown < 93){document.getElementById("braSize").value = 90}

            

            if(result < 6 || result > 21){alert("对不起，没有适合您的罩杯！");}

            else if(result < 8){document.getElementById("braClass").value = "AA"}

            else if(result < 10){document.getElementById("braClass").value = "A"}

            else if(result < 13.5){document.getElementById("braClass").value = "B"}

            else if(result < 16){document.getElementById("braClass").value = "C"}

            else if(result < 20){document.getElementById("braClass").value = "D"}

            }

        }

        

    function underwearSize_onClick(){                                          //内裤尺码测量函数

        var waistSize = document.getElementById("waistSize").value;

        var hipSize = document.getElementById("hipSize").value;

        if(waistSize != "" && hipSize != ""){

            if(waistSize < 58 || waistSize > 83 || hipSize < 79 || hipSize > 103){alert("对不起！没有适合您的尺码。请重输");}

            if(waistSize >=58 && waistSize <=63){if(hipSize>=79 && hipSize<=89) document.getElementById("underwearSize").value = "S";}

            if(waistSize >=63 && waistSize <=68){if(hipSize>=83 && hipSize<=93) document.getElementById("underwearSize").value = "M";}

            if(waistSize >=68 && waistSize <=73){if(hipSize>=86 && hipSize<=96) document.getElementById("underwearSize").value = "L";}

            if(waistSize >=73 && waistSize <=78){if(hipSize>=89 && hipSize<=99) document.getElementById("underwearSize").value = "XL";}

            if(waistSize >=78 && waistSize <=83){if(hipSize>=91 && hipSize<=103) document.getElementById("underwearSize").value = "XXL";}

            }

        }

</script>