Switch 语句

Switch 语句将表达式的值与 1 个或多个值进行比较，并根据该比较执行不同的代码段。

var value = 1;
switch (value) {
  case 1:
    console.log('I will always run');
    break;
  case 2:
    console.log('I will never run');
    break;
}

break 语句从 switch 语句中中断并确保不再执行 switch 语句中的代码。这就是如何定义部分并允许用户进行中断的情况。

警告 ：每个案例缺少 break 或 return 声明意味着程序将继续评估下一个案例，即使案例标准未得到满足！

switch (value) {
  case 1:
    console.log('I will only run if value === 1');
    // Here, the code "falls through" and will run the code under case 2
  case 2:
    console.log('I will run if value === 1 or value === 2');
    break;
  case 3:
    console.log('I will only run if value === 3');
    break;
}

最后一个案例是 default 情况。如果没有其他匹配，这个将运行。

var animal = 'Lion';
switch (animal) {
  case 'Dog':
    console.log('I will not run since animal !== "Dog"');
    break;
  case 'Cat':
    console.log('I will not run since animal !== "Cat"');
    break;
  default:
    console.log('I will run since animal does not match any other case');
}

应该注意，案例表达可以是任何类型的表达。这意味着你可以使用比较，函数调用等作为案例值。

function john() {
  return 'John';
}

function jacob() {
  return 'Jacob';
}

switch (name) {
  case john(): // Compare name with the return value of john() (name == "John")
    console.log('I will run if name === "John"');
    break;
  case 'Ja' + 'ne': // Concatenate the strings together then compare (name == "Jane")
    console.log('I will run if name === "Jane"');
    break;
  case john() + ' ' + jacob() + ' Jingleheimer Schmidt':
    console.log('His name is equal to name too!');
    break;
}

case 的多重包容性标准

由于 case 在没有 break 或 return 语句的情况下掉头，你可以使用它来创建多个包含标准：

var x = "c"
switch (x) {
   case "a":
   case "b":
   case "c":
      console.log("Either a, b, or c was selected.");
      break;
   case "d":
      console.log("Only d was selected.");
      break;
   default:
      console.log("No case was matched.");
      break;  // precautionary break if case order changes
}