Assign variable in if condition statement, good practice or not? [closed]

Question

I wouldn't recommend it. The problem is, it looks like a common error where you try to compare values, but use a single = instead of == or ===. For example, when you see this:

if (value = someFunction()) {
    ...
}

you don't know if that's what they meant to do, or if they intended to write this:

if (value == someFunction()) {
    ...
}

If you really want to do the assignment in place, I would recommend doing an explicit comparison as well:

if ((value = someFunction()) === <whatever truthy value you are expecting>) {
    ...
}

I see no proof that it is not good practice. Yes, it may look like a mistake but that is easily remedied by judicious commenting. Take for instance:

if (x = processorIntensiveFunction()) { // declaration inside if intended
    alert(x);
}

Why should that function be allowed to run a 2nd time with:

alert(processorIntensiveFunction());

Because the first version LOOKS bad? I cannot agree with that logic.

I did it many times. To bypass the JavaScript warning, I add two parens:

if ((result = get_something())) { }

You should avoid it, if you really want to use it, write a comment above it saying what you are doing.

There is one case when you do it, with while-loops.
When reading files, you usualy do like this:

void readFile(String pathToFile) {
    // Create a FileInputStream object
    FileInputStream fileIn = null;
    try {
        // Create the FileInputStream
        fileIn = new FileInputStream(pathToFile);
        // Create a variable to store the current line's text in
        String currentLine;
        // While the file has lines left, read the next line,
        // store it in the variable and do whatever is in the loop
        while((currentLine = in.readLine()) != null) {
            // Print out the current line in the console
            // (you can do whatever you want with the line. this is just an example)
            System.out.println(currentLine);
        }
    } catch(IOException e) {
        // Handle exception
    } finally {
        try {
            // Close the FileInputStream
            fileIn.close();
        } catch(IOException e) {
            // Handle exception
        }
    }
}

Look at the while-loop at line 9. There, a new line is read and stored in a variable, and then the content of the loop is ran. I know this isn't an if-statement, but I guess a while loop can be included in your question as well.

The reason to this is that when using a FileInputStream, every time you call FileInputStream.readLine(), it reads the next line in the file, so if you would have called it from the loop with just fileIn.readLine() != null without assigning the variable, instead of calling (currentLine = fileIn.readLine()) != null, and then called it from inside of the loop too, you would only get every second line.

Hope you understand, and good luck!