Question

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.

Answer 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.
}