问题
Sorry for that dumb question.
How can I use a condition for a case in the javascript switch-case language element?
Like in the example below, a case should match when the variable liCount is <=5 and >0; however, my code does not work:
switch (liCount) {
case 0:
setLayoutState(\'start\');
var api = $(\'#UploadList\').data(\'jsp\');
api.reinitialise();
break;
case (liCount<=5 && liCount>0):
setLayoutState(\'upload1Row\');
var api = $(\'#UploadList\').data(\'jsp\');
api.reinitialise();
break;
case (liCount<=10 && liCount>5):
setLayoutState(\'upload2Rows\');
var api = $(\'#UploadList\').data(\'jsp\');
api.reinitialise();
break;
case (liCount>10):
var api = $(\'#UploadList\').data(\'jsp\');
api.reinitialise();
break;
}
Thanks for any advice!
回答1:
This works:
switch (true) {
case liCount == 0:
setLayoutState('start');
var api = $('#UploadList').data('jsp');
api.reinitialise();
break;
case liCount<=5 && liCount>0:
setLayoutState('upload1Row');
var api = $('#UploadList').data('jsp');
api.reinitialise();
break;
case liCount<=10 && liCount>5:
setLayoutState('upload2Rows');
var api = $('#UploadList').data('jsp');
api.reinitialise();
break;
case liCount>10:
var api = $('#UploadList').data('jsp');
api.reinitialise();
break;
}
A previous version of this answer considered the parentheses to be the culprit. In truth, the parentheses are irrelevant here - the only thing necessary is switch(true){...} and for your case expressions to evaluate to booleans.
It works because, the value we give to the switch is used as the basis to compare against. Consequently, the case expressions, also evaluating to booleans will determine which case is run. Could also turn this around, and pass switch(false){..} and have the desired expressions evaluate to false instead of true.. but personally prefer dealing with conditions that evaluate to truthyness. However, it does work too, so worth keeping in mind to understand what it is doing.
Eg: if liCount is 3, the first comparison is true === (liCount == 0), meaning the first case is false. The switch then moves on to the next case true === (liCount<=5 && liCount>0). This expression evaluates to true, meaning this case is run, and terminates at the break. I've added parentheses here to make it clearer, but they are optional, depending on the complexity of your expression.
It's pretty simple, and a neat way (if it fits with what you are trying to do) of handling a long series of conditions, where perhaps a long series of ìf() ... else if() ... else if () ... might introduce a lot of visual noise or fragility.
Use with caution, because it is a non-standard pattern, despite being valid code.
回答2:
You've way overcomplicated that. Write it with if statements instead like this:
if(liCount == 0)
setLayoutState('start');
else if(liCount<=5)
setLayoutState('upload1Row');
else if(liCount<=10)
setLayoutState('upload2Rows');
$('#UploadList').data('jsp').reinitialise();
Or, if ChaosPandion is trying to optimize as much as possible:
setLayoutState(liCount == 0 ? 'start' :
liCount <= 5 ? 'upload1Row' :
liCount <= 10 ? 'upload2Rows' :
null);
$('#UploadList').data('jsp').reinitialise();
回答3:
You want to use if statements:
if (liCount === 0) {
setLayoutState('start');
} else if (liCount <= 5) {
setLayoutState('upload1Row');
} else if (liCount <= 10) {
setLayoutState('upload2Rows');
}
$('#UploadList').data('jsp').reinitialise();
回答4:
See dmp's answer below. I'd delete this answer if I could, but it was accepted so this is the next best thing :)
You can't. JS Interpreters require you to compare against the switch statement (e.g. there is no "case when" statement). If you really want to do this, you can just make if(){ .. } else if(){ .. } blocks.
回答5:
switch (true) {
case condition0:
...
break;
case condition1:
...
break;
}
will work in JavaScript as long as your conditions return proper boolean values, but it doesn't have many advantages over else if statements.
回答6:
If that's what you want to do, it would be better to use if statements. For example:
if(liCount == 0){
setLayoutState('start');
}
if(liCount<=5 && liCount>0){
setLayoutState('upload1Row');
}
if(liCount<=10 && liCount>5){
setLayoutState('upload2Rows');
}
var api = $('#UploadList').data('jsp');
api.reinitialise();
回答7:
That's a case where you should use if clauses.
回答8:
Your code does not work because it is not doing what you are expecting it to do. Switch blocks take in a value, and compare each case to the given value, looking for equality. Your comparison value is an integer, but most of your case expressions resolve to a boolean value.
So, for example, say liCount = 2. Your first case will not match, because 2 != 0. Your second case, (liCount<=5 && liCount>0) evaluates to true, but 2 != true, so this case will not match either.
For this reason, as many others have said, you should use a series of if...then...else if blocks to do this.
回答9:
if the possible values are integers you can bunch up cases. Otherwise, use ifs.
var api, tem;
switch(liCount){
case 0:
tem= 'start';
break;
case 1: case 2: case 3: case 4: case 5:
tem= 'upload1Row';
break;
case 6: case 7: case 8: case 9: case 10:
tem= 'upload2Rows';
break;
default:
break;
}
if(tem) setLayoutState((tem);
api= $('#UploadList').data('jsp');
api.reinitialise();
回答10:
Notice that we don't pass score to the switch but true. The value we give to the switch is used as the basis to compare against.
The below example shows how we can add conditions in the case: without any if statements.
function getGrade(score) {
let grade;
// Write your code here
switch(true) {
case score >= 0 && score <= 5:
grade = 'F';
break;
case score > 5 && score <= 10:
grade = 'E';
break;
case score > 10 && score <= 15:
grade = 'D';
break;
case score > 15 && score <= 20:
grade = 'C';
break;
case score > 20 && score <= 25:
grade = 'B';
break;
case score > 25 && score <= 30:
grade = 'A';
break;
}
return grade;
}
来源：https://stackoverflow.com/questions/5464362/javascript-using-a-condition-in-switch-case