switch

概述

switch语句对一个表达式求值，将结果与 case 子语句比较，如果匹配，则从 case 处的语句向下执行。

语法

switch (expression) {
  case value1:
    // 当 expression 的结果与 value1 匹配时，从此处开始执行
    statements1；
    [break;]
  case value2:
    // 当 expression 的结果与 value2 匹配时，从此处开始执行
    statements2;
    [break;]
  ...
  case valueN:
    // 当 expression 的结果与 valueN 匹配时，从此处开始执行
    statementsN;
    [break;]
  default:
    // 如果 expression 与上面的 value 值都不匹配时，执行此处的语句
    statements_def;
    [break;]
}

expression

一个用来与 case 子语句匹配的表达式。

case expressionN

case 子语句，用来与 expression 匹配。

statementsN

如果 expression 匹配其对应的 case 子语句，则执行此处的语句。

statements_def

如果 expression 没有匹配到任何 case 子语句，则会执行此处的语句。

描述

If a match is found, the program executes the associated statements. If multiple cases match the provided value, the first case that matches is selected, even if the cases are not equal to each other.

The program first looks for a case clause whose expression evaluates to the same value as the input expression (using strict comparison, ===) and then transfers control to that clause, executing the associated statements. If no matching case clause is found, the program looks for the optional default clause, and if found, transfers control to that clause, executing the associated statements. If no default clause is found, the program continues execution at the statement following the end of switch. By convention, the default clause is the last clause, but it does not need to be so.

The optional break statement associated with each case label ensures that the program breaks out of switch once the matched statement is executed and continues execution at the statement following switch. If break is omitted, the program continues execution at the next statement in the switch statement.

示例

示例1：使用 switch

In the following example, if expr evaluates to "Bananas", the program matches the value with case "Bananas" and executes the associated statement. When break is encountered, the program breaks out of switch and executes the statement following switch. If break were omitted, the statement for case "Cherries" would also be executed.

switch (expr) {
  case "Oranges":
    console.log("Oranges are $0.59 a pound.");
    break;
  case "Apples":
    console.log("Apples are $0.32 a pound.");
    break;
  case "Bananas":
    console.log("Bananas are $0.48 a pound.");
    break;
  case "Cherries":
    console.log("Cherries are $3.00 a pound.");
    break;
  case "Mangoes":
  case "Papayas":
    console.log("Mangoes and papayas are $2.79 a pound.");
    break;
  default:
    console.log("Sorry, we are out of " + expr + ".");
}

console.log("Is there anything else you'd like?");

示例2：如果忘记中断(break)会怎么样？

If you forget a break then script will run from the case where criteria is met, and will run the case after that regardless if criteria was met. See example here:

var foo = 0;
switch (foo) {
    case -1:
        console.log('negative 1');
        break;
    case 0: // foo is 0 so criteria met here so this block will run
        console.log(0)
        // NOTE: the forgotten break would have been here
    case 1: // no break statement in 'case 0:' so this case will run as well
        console.log(1);
        break; // it encounters this break so will not continue into 'case 2:'
    case 2:
        console.log(2);
        break;
    default:
        console.log('default');
}

示例：使用多条件 case 的方法

If you would like one case segment to meet multiple criteria there are two methods/tricks. Method one is the recommended way, method two is a "hacky" way. The two methods below aim to alert "yes" when variable foo is 0, 1, 2, or 3.

Sources for these techniques are here:

1. Multiple Criteria Single Case Switch Statement (Stack Overflow)
2. Switch statement multiple cases in JavaScript (Stack Overflow)

方法一

This method takes advantage of the fact that if there is no break below a case statement it will continue to execute the next case statement regardless if the case meets the criteria. See the section title "What happens if I forgot a break?"

var foo = 1;
switch (foo) {
    case 0:
    case 1:
    case 2:
    case 3:
        alert('yes');
        break;
    default:
        alert('not');
}

方法二

This is the "hacky" method; on line 2, where you would usually find switch(foo), we instead put an invariant true, and place conditional logic (instead of constants) into the case statements:

var foo = 1;
switch (true) { // 非变量 TRUE 替代 foo
    case foo >= 0 && foo <= 3:
        alert('yes');
        break;
    default:
        alert('not');
}

规范

浏览器兼容性

相关链接

• if...else