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

I moved one years ago from classic OO languages such like Java to JavaScript. The following code is definitely not recommended (or even not correct) in Java:

if(dayNumber = getClickedDayNumber(dayInfo))
{
    alert("day number found : " + dayNumber);
}
function getClickedDayNumber(dayInfo)
{
    dayNumber = dayInfo.indexOf("fc-day");
    if(dayNumber != -1) //substring found
    {
        //normally any calendar month consists of "40" days, so this will definitely pick up its day number.
        return parseInt(dayInfo.substring(dayNumber+6, dayNumber+8));
    }
    return false;
}

Basically I just found out that I can assign a variable to a value in an if condition statement, and immediately check the assigned value as if it is boolean.

For a safer bet, I usually separate that into two lines of code, assign first then check the variable, but now that I found this, I am just wondering whether is it good practice or not in the eyes of experienced JavaScript developers?

Answer

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>) {
    ...
}