JavaScript Switch Statement
这 switch statement is used to perform different actions based on different conditions.
The JavaScript Switch Statement
使用 switch statement to select one of many code blocks to be executed.
Syntax
switch(expression) {
case x:
// code block
break;
case y:
// code block
break;
default:
// code block
}
This is how it works:
- The switch expression is evaluated once.
- The value of the expression is compared with the values of each case.
- If there is a match, the associated block of code is executed.
- If there is no match, the default code block is executed.
例子
这 getDay() method returns the weekday as a number between 0 and 6.
(Sunday=0, Monday=1, Tuesday=2 ..)
This example uses the weekday number to calculate the weekday name:
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";
}
The result of day will be:
The break Keyword
When JavaScript reaches a break keyword, it breaks out of the switch block.
This will stop the execution inside the switch block.
It is not necessary to break the last case in a switch block. The block breaks (ends) there anyway.
Note: If you omit the break statement, the next case will be executed even if the evaluation does not match the case.
The default Keyword
这 default keyword specifies the code to run if there is no case match:
例子
这 getDay() method returns the weekday as a number between 0 and 6.
If today is neither Saturday (6) nor Sunday (0), write a default message:
switch (new Date().getDay()) {
case 6:
text = "Today is Saturday";
break;
case 0:
text = "Today is Sunday";
break;
default:
text = "Looking forward to the Weekend";
}
The result of text will be:
这 default case does not have to be the last case in a switch block:
例子
switch (new Date().getDay()) {
default:
text = "Looking forward to the Weekend";
break;
case 6:
text = "Today is Saturday";
break;
case 0:
text = "Today is Sunday";
}
亲自尝试 »
If default is not the last case in the switch block, remember to end the default case with a break.
Common Code Blocks
Sometimes you will want different switch cases to use the same code.
In this example case 4 and 5 share the same code block, and 0 and 6 share another code block:
例子
switch (new Date().getDay()) {
case 4:
case 5:
text = "Soon it is Weekend";
break;
case 0:
case 6:
text = "It is Weekend";
break;
default:
text = "Looking forward to the Weekend";
}
亲自尝试 »
Switching Details
If multiple cases matches a case value, the first case is selected.
If no matching cases are found, the program continues to the default label.
If no default label is found, the program continues to the statement(s) after the switch.
Strict Comparison
Switch cases use strict comparison (===).
The values must be of the same type to match.
A strict comparison can only be true if the operands are of the same type.
In this example there will be no match for x:
例子
let x = "0";
switch (x) {
case 0:
text = "Off";
break;
case 1:
text = "On";
break;
default:
text = "No value found";
}
亲自尝试 »
