是什么导致错误“uncaughttypeerror:number不是函数”

我有一个更新表单的onchange事件,在更新过程中它调用一个函数来计算运费。我不知道为什么,但在尝试调用函数时出现以下错误:

Uncaught TypeError: number is not a function

shipping ,如下所示:

function shipping( weight )
{
    var flat

    switch( weight )
    {
        case 1:
        case 2:
        case 3:
            flat = 32.00;
            break;

        case 4:
            flat = 18.50;
            break;

        case 5:
            flat = 15.80;
            break;

        case 6:
            flat = 14.00;
            break;

        case 7:
            flat = 12.71;
            break;

        case 8:
            flat = 11.75;
            break;

        case 9:
            flat = 11.00;
            break;

        case 10:
            flat = 10.40;
            break;

        case 11:
            flat = 9.91;
            break;

        case 12:
            flat = 9.50;
            break;

        case 13:
            flat = 9.15;
            break;

        case 14:
            flat = 8.86;
            break;

        case 15:
            flat = 8.86;
            break;

        case 16:
            flat = 8.38;
            break;

        case 17:
            flat = 8.18;
            break;

        case 18:
            flat = 8.00;
            break;

        case 19:
            flat = 7.84;
            break;

        case 20:
            flat = 7.70;
            break;

    } // switch

    var flat_fee = flat * weight;
    var mile_fee = distance * 0.90;

    var shipping_fee = flat_fee + mile_fee;
    simpleCart.shippingTest = shipping_fee;
    return shipping_fee;
} // shipping

我要过去 1 马上。变量 distance 来自在运行此函数之前完成的ajax调用。该函数如下所示:

function get_distance( zip )
{

    $.getJSON(
        'distance.php',
        { zip:zip },
        function(json)
        {
            distance = json 
        })

} // get_distance

我已经检查了变量 已设置。

控制台说在我呼叫的线路上发生了未捕获的类型错误 shipping(1) . 你有没有想过为什么会这样?