Javascript If statement used to check file extensi

问题:

I have a form in which people can enter file paths. I want to make sure that the paths they are entering point to pictures so here is what I thought would work.

function checkExt()
{
    var extension= /* I know that the code to cut the extension off of the file
                      is working correctly so for now let's just go with it ok */
    if(extension!="jpg" || "gif" || "bmp" || "png" || "whatever else")
        alert("The file extension you have entered is not supported");
}

But this does not work. I have tracked it down to the if statement because if I select only 1 kind of file to check for, then it will work correctly. So my question to you is what the hell do I have to change to make this thing work correctly. I've been on this for about three hours now and it's driving me mad. Thanks for all of the help in advance.

回答1:

That's a syntax and a logic error. It should be:

if (extension != "jpg" && 
    extension != "gif" && 
    extension != "bmp" && 
    extension != "png" && 
    extension != "whatever else") {
    // This will execute when the extension is NOT one of the expected 
    // extensions.
}

Furthermore, you could handle it a little more succinctly with a regular expression:

if (!/jpg|gif|bmp|png|whatever/.test(extension)) {
    // This will execute when the extension is NOT one of the expected 
    // extensions.
}

Addendum:

The examples above execute the body of the if-statement when the value of extension is not one of the supported values. If you wanted to execute the body of the if-statement when the value of extensions is one of the supported values, you would change the logic from not equal/and to equal/or, like so:

if (extension == "jpg" || 
    extension == "gif" || 
    extension == "bmp" || 
    extension == "png" || 
    extension == "whatever else") {
    // This will execute when the extension is one of the expected extensions.
}

And again, it'd be more concise using a regular expression:

// I just removed the leading ! from the test case.
if (/jpg|gif|bmp|png|whatever/.test(extension)) {
    // This will execute when the extension is one of the expected extensions.
}

回答2:

As an alternative to the verbosity, use a regex:

if (extension.search(/jpg|gif|bmp|png/) === -1) {
    alert("The file extension you have entered is not supported.");
}

回答3:

You should be using && instead of || and you must prefix the != operator with extension on each condition, not just the first one:

if (extension != "jpg" &&
    extension != "gif" &&
    extension != "bmp" && 
    extension != "png" &&
    extension != "whatever else")

回答4:

You need to include the parameters to compare in each clause.

if( extension != "jpg" || 
    extension != "gif" || 
    extension != "bmp" || 
    extension != "png" || 
    extension != "whatever else"){
 //TODO: handle condition
}

As others have noted, this is a logical error in that if "jpg" is the extension the condition will still be hit because "jpg" != "gif". You might want to consider using &&.

回答5:

You have to compare the variable extension for each tested extension. A compact way to do this would be:

var extension = whatever;
var supportedExtensions = ["jpg","gif","bmp","png",... whatever else];
if (supportedExtensions.indexOf(extension) < 0) alert("Unsupported extension");

回答6:

The other answers have shown you how your javascript logic for multiple comparisons is wrong. But, a much better way to do this is to use a javascript map like this:

function checkExt() {
    var allowedExtensions = {
        "jpg": true, "gif": true, "bmp": true, "png": true, "whatever else":true
    };
    var extension= /* I know that the code to cut the extension off of the file
                      is working correctly so for now let's just go with it ok */
    if (allowedExtensions[extension] !== true) {
        alert("The file extension you have entered is not supported");
    }
}

The is a easier to maintain by just adding/removing items to/from the allowedExtensions object.