在条件陈述中分配变量，良好实践与否？ [英] Assign variable in if condition statement, good practice or not?

问题描述

一年前，我将经典的OO语言（如Java）转移到JavaScript。 Java中绝对不推荐使用以下代码（甚至不正确）：

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));
    }
    else return false;
}

基本上我发现我可以为一个值分配一个变量如果条件语句，并立即检查指定的值，就好像它是布尔值。

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.

为了更安全的赌注，我通常将其分成两行代码，首先分配然后检查变量，但是现在我发现了这个，我只是想知道在有经验的JavaScript开发人员看来它是否是好的做法？

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?

推荐答案

我不推荐它。问题是，在您尝试比较值时，它看起来像是一个常见错误，但使用单个 = 而不是 == 或 === 。例如，当你看到这个：

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()) {
    ...
}

你不知道这是他们的意图，还是他们打算写这个：

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

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

如果你真的想做适当的任务，我建议你做一个明确的比较：

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>) {
    ...
}

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