switch

switch statement evaluates an expression , matching the expression's value to a case clause, and executes statements associated with that case , as well as statements in case s that follow the matching case .

句法

switch (expression) {
  case value1:
    //Statements executed when the
    //result of expression matches value1
    [break;]
  case value2:
    //Statements executed when the
    //result of expression matches value2
    [break;]
  ...
  case valueN:
    //Statements executed when the
    //result of expression matches valueN
    [break;]
  [default:
    //Statements executed when none of
    //the values match the value of the expression
    [break;]]
}
					
expression
An expression whose result is matched against each case clause.
case valueN 可选
A case clause used to match against expression 。若 expression matches the specified valueN , the statements inside the case clause are executed until either the end of the switch statement or a break .
default 可选
A default clause; if provided, this clause is executed if the value of expression doesn't match any of the case clauses.

描述

A switch statement first evaluates its expression. It then looks for the first case clause whose expression evaluates to the same value as the result of the input expression (using the strict comparison , === ) and transfers control to that clause, executing the associated statements. (If multiple case s match the provided value, the first case that matches is selected, even if the case s are not equal to each other.)

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.

可选 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 。若 break is omitted, the program continues execution at the next statement in the switch statement. The break statement is not required if a return statement precedes it.

范例

使用 switch

In the following example, if expr evaluates to Bananas , the program matches the value with case case 'Bananas' and executes the associated statement. When break is encountered, the program breaks out of switch and executes the statement following switch 。若 break were omitted, the statement for the 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?");
					

What happens if I forgot a break ?

If you forget a break then the script will run from the case where the criterion is met and will run the cases after that regardless if a criterion 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');
}
					

Can I put a default between cases?

Yes, you can! JavaScript will drop you back to the default if it can't find a match:

var foo = 5;
switch (foo) {
  case 2:
    console.log(2);
    break; // it encounters this break so will not continue into 'default:'
  default:
    console.log('default')
    // fall-through
  case 1:
    console.log('1');
}
					

It also works when you put default before all other case s.

Methods for multi-criteria case

Source for this technique is here:

Switch statement multiple cases in JavaScript (Stack Overflow)

Multi- case : single operation

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

This is an example of a single operation sequential case statement, where four different values perform exactly the same.

var Animal = 'Giraffe';
switch (Animal) {
  case 'Cow':
  case 'Giraffe':
  case 'Dog':
  case 'Pig':
    console.log('This animal will go on Noah\'s Ark.');
    break;
  case 'Dinosaur':
  default:
    console.log('This animal will not.');
}
					

Multi- case : chained operations

This is an example of a multiple-operation sequential case clause, where, depending on the provided integer, you can receive different output. This shows you that it will traverse in the order that you put the case clauses, and it does not have to be numerically sequential. In JavaScript, you can even mix in definitions of strings into these case statements as well.

var foo = 1;
var output = 'Output: ';
switch (foo) {
  case 0:
    output += 'So ';
  case 1:
    output += 'What ';
    output += 'Is ';
  case 2:
    output += 'Your ';
  case 3:
    output += 'Name';
  case 4:
    output += '?';
    break;
  case 5:
    output += '!';
    console.log(output);
    break;
  default:
    console.log('Please pick a number from 0 to 5!');
}
					

The output from this example:

Log text
foo is NaN or not 1 , 2 , 3 , 4 , 5 ,或 0 Please pick a number from 0 to 5!
0 Output: So What Is Your Name?
1 Output: What Is Your Name?
2 Output: Your Name?
3 Output: Name?
4 Output: ?
5 Output: !

Block-scope variables within switch statements

With ECMAScript 2015 (ES6) support made available in most modern browsers, there will be cases where you would want to use let and const statements to declare block-scoped variables.

Take a look at this example:

const action = 'say_hello';
switch (action) {
  case 'say_hello':
    let message = 'hello';
    console.log(message);
    break;
  case 'say_hi':
    let message = 'hi';
    console.log(message);
    break;
  default:
    console.log('Empty action received.');
    break;
}
					

This example will output the error Uncaught SyntaxError: Identifier 'message' has already been declared which you were not probably expecting.

This is because the first let message = 'hello'; conflicts with second let statement let message = 'hi'; even they're within their own separate case clauses case 'say_hello': and case 'say_hi': . Ultimately, this is due to both let statements being interpreted as duplicate declarations of the same variable name within the same block scope.

We can easily fix this by wrapping our case clauses with brackets:

const action = 'say_hello';
switch (action) {
  case 'say_hello': { // added brackets
    let message = 'hello';
    console.log(message);
    break;
  } // added brackets
  case 'say_hi': { // added brackets
    let message = 'hi';
    console.log(message);
    break;
  } // added brackets
  default: { // added brackets
    console.log('Empty action received.');
    break;
  } // added brackets
}
					

This code will now output hello in the console as it should, without any errors at all.

规范

规范
ECMAScript (ECMA-262)
The definition of 'switch statement' in that specification.

浏览器兼容性

更新 GitHub 上的兼容性数据
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图例

完整支持
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另请参阅

Metadata

  1. JavaScript
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    41. SyntaxError: redeclaration of formal parameter "x"
    42. SyntaxError: return not in function
    43. SyntaxError: test for equality (==) mistyped as assignment (=)?
    44. SyntaxError: unterminated string literal
    45. TypeError: "x" has no properties
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    50. TypeError: "x" is read-only
    51. TypeError: 'x' is not iterable
    52. TypeError: More arguments needed
    53. TypeError: Reduce of empty array with no initial value
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    55. TypeError: can't access dead object
    56. TypeError: can't access property "x" of "y"
    57. TypeError: can't assign to property "x" on "y": not an object
    58. TypeError: can't define property "x": "obj" is not extensible
    59. TypeError: can't delete non-configurable array element
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    61. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    62. TypeError: cyclic object value
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    64. TypeError: invalid Array.prototype.sort argument
    65. TypeError: invalid arguments
    66. TypeError: invalid assignment to const "x"
    67. TypeError: property "x" is non-configurable and can't be deleted
    68. TypeError: setting getter-only property "x"
    69. TypeError: variable "x" redeclares argument
    70. URIError: malformed URI sequence
    71. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    72. Warning: 08/09 is not a legal ECMA-262 octal constant
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    74. Warning: JavaScript 1.6's for-each-in loops are deprecated
    75. Warning: String.x is deprecated; use String.prototype.x instead
    76. Warning: expression closures are deprecated
    77. Warning: unreachable code after return statement
  13. 杂项
    1. JavaScript technologies overview
    2. 词汇语法
    3. JavaScript 数据结构
    4. Enumerability and ownership of properties
    5. Iteration protocols
    6. 严格模式
    7. Transitioning to strict mode
    8. Template literals
    9. 弃用特征