Switch语句不适用于某些变量

Question

I want to make the Angular2 Pipe which returns different values according to passed variable. The variable device.display can be of different type (object or string) and takes only two values null and "dataTime" .

Component part:

<div class="col col-2">{{values[variable.id] | display: device.display}}</div>

Pipe:

Pipe
import { Pipe } from '@angular/core';
@Pipe({
    name: 'display'
})
export class DisplayPipe {
    transform(value: any, display: string): string {
        display = String(display);
        switch (display) {
            case 'dateTime': {
                value = value * 60;
            }
            case 'null': {
                value = value;
           }
        default: value = 'abc';
        }
    return value;
    }
}

I'm sure that after parsing variable to String it is equal to "null" or "dateTime" . I checked it with console.log .

The problem is the switch statement always returns default value: "abc" .

Answer 1

Wrong Syntax. You don't need brackets in each case, and you forgot the break; statement.

Heres an example, on how you build switch case:

switch (new Date().getDay()) {
    case 0:
        day = "Sunday";
        break;
    case 1:
        day = "Monday";
        break;
    case 2:
        day = "Tuesday";
        break;
    case 3:
        day = "Wednesday";
        break;
    case 4:
        day = "Thursday";
        break;
    case 5:
        day = "Friday";
        break;
    case 6:
        day = "Saturday";
}

Answer 2

That's not correct sintaxis for switch:

switch (display) {
            case 'dateTime':
                value = value * 60;
                break;
            case 'null':
                value = value;
                break;
            default: 
               value = 'abc';
        }

if you dont break on every case, switch will execute all cases, then how you are setting same variable on all cases, the value is the last case default: abc.

Answer 3

如果您错过了Java中的break语句， 则会发生穿透情况，因此将执行默认语句，并且值始终为“ abc”