为什么我的jquery switch语句不起作用？

Question

I am trying a jquery (javascript) switch statement with some simple operators but it isn't working as expecting

console.log('test '+getShippingCost(8));
function getShippingCost(shop_qty) {
                shipping_costs = 0;
                dest = $("input[name='dest']").val();
                console.log(dest);
                if (dest === 'DOMESTIC') {

                    switch (shop_qty) {
                        case (shop_qty > 4):
                             shipping_costs = 3.5;
                            break;
                        case (shop_qty <= 4):
                             shipping_costs = 2;
                                break;
                        }
                        console.log('domestic shipping '+shipping_costs);
                }
                if (dest === 'INT') {
                            switch (shop_qty) {
                                case (shop_qty > 4):
                                    shipping_costs = 4.5;
                                    break;
                                case (shop_qty <= 4):
                                    shipping_costs = 3;
                                    break;
                            }
                        }

                        return shipping_costs;
                        }//end function

see see jsfiddle

Answer 1

To use conditions for the cases in a switch , you would look for a true value among the cases:

switch (true) {
  case (shop_qty > 4):
    shipping_costs = 3.5;
    break;
  case (shop_qty <= 4):
    shipping_costs = 2;
    break;
}

As the second case is the inverse of the first, you can just use default for that:

switch (true) {
  case (shop_qty > 4):
    shipping_costs = 3.5;
    break;
  default:
    shipping_costs = 2;
    break;
}

Such a construct fits better when you have several conditions. You should consider if an if statement would be a better fit in this case:

if (shop_qty > 4) {
    shipping_costs = 3.5;
} else {
    shipping_costs = 2;
}

As both cases assign a value to the same variable, you can also write it using the conditional operator:

shipping_costs = shop_qty > 4 ? 3.5 : 2;

Answer 2

Cases in switch statements evaluate values, not boolean expressions: See Here .

You can put the logic suggested by your attempted use of the switch statement in a ternary expression, eg:

shipping_costs = (shop_qty > 4) ? 3.5 : 2;

Answer 3

Switch doesn't work by evaluating boolean expressions. Switch evaluates an initial expression and then tries to match the cases to that expression using strict equality. So you can't do

case(x<4.5):

You have to do something like

case 4:

Use if, else if, else statments instead.