如何在 javascript 开关盒 function 中使用 !-1

Question

I "want" to use switch, but i can't seem to get it to work when I use it in this fashion. Anyone know why?

var mystring='my name is johnny';  
switch (!-1) {  
          case mystring.indexOf('hello'):  
        alert('hello');  
        break;  
    case mystring.indexOf('goodbye'):  
        alert('goodbye');  
        break;  
    case mystring.indexOf('johnny'):  
        alert('johnny');  
        break;  
    default:  
        alert('default');           
}  

it always alerts "default", but you can see that I want it to alert "johnny"

Answer 1

Disclaimer: This switch is evil . Use if/else-if statements. but if you must use a switch it can be done as such:

switch (true) {  
    case /hello/.test(mystring):  
        alert('hello');  
        break;  
    case /goodbye/.test(mystring):  
        alert('goodbye');  
        break;  
    case /johnny/.test(mystring):  
        alert('johnny');  
        break;  
    default:  
        alert('default');           
}  

Should work as you want it to.

.test .

I would be tempted to refactor it further.

function Parse(str) {
    var logic = {
        "hello": function(str) { alert("hello"); }
        /*, ... */
    }

    for (var k in logic) {
        if ((new RegExp(k)).test(str)) {
            logic[k](str);
        }
    }
}

Parse("Hello I am johnny");

Answer 2

!-1 coerces -1 to a boolean, and then negates it, so

switch(!-1)
{
    // stuff
}

is equivalent to

switch(false)
{
    // stuff
}

Since there is no case false , the default will always be executed. This is really not the right way to use a switch — there is no point in switch ing on a constant expression.

Why do you "want" to use a switch ?

---

Here's how to implement it correctly:

var mystring='my name is johnny';

if (mystring.indexOf('hello') !== -1) alert('hello');
else if (mystring.indexOf('goodbye') !== -1) alert('goodbye');
else if (mystring.indexOf('johnny') !== -1) alert('johnny');

Or, less repetetively,

var mystring='my name is johnny',
    words = ['hello', 'goodbye', 'johnny'],
    word;

while (word = words.shift(), mystring.indexOf(word) === -1);

word = word || 'default';
alert(word);

Answer 3

Your combination of "not"( ! ) and " -1 " seems faulty. You need to move them to your cases like this

var mystring='my name is johnny';  
switch (true) {  
          case mystring.indexOf('hello') != -1:  
        alert('hello');  
        break;  
    case mystring.indexOf('goodbye') != -1:  
        alert('goodbye');  
        break;  
    case mystring.indexOf('johnny') != -1:  
        alert('johnny');  
        break;  
    default:  
        alert('default');           
}

It's a pain in PHP that strpos(...) returns false when not found and 0 when string starts with it (and those 2 values are == , but not === ) but I personally find JavaScripts indexOf(..) == -1 even worse. (Why can't it be false or null ?)

Answer 4

What's happening here is you're switch ing the result of !-1 and that case isn't found. switch-case doesn't seem to be the right construct to use here.

Answer 5

The expression !-1 will just evaluate to false .

You are trying to convert a comparison like x != -1 into x == !-1 , but the ! operator doesn't negate the comparison, only the value.

Answer 6

I don't think your implementation is correct. The proper conditional statement to use is if... else .

var mystring='my name is johnny';

if(mystring.indexOf('hello');= -1) alert('hello');
else if(mystring.indexOf('goodbye');= -1) alert('goodbye');
else if(mystring.indexOf('johnny');= -1) alert('johnny');
else alert('default');

Answer 7

The switch should be mystring, example:

http://www.w3schools.com/js/js_switch.asp