javascript - 减去两个 HH :MM strings with a recursive function

Question

我一直在尝试获取 this上类，我找到了here并根据我的需要进行调整。目前它正在达到预期的值，但是返回值不会在返回时传递到函数之外。我试过对其进行调试，但似乎无法找到/理解问题所在。

该函数应该减去格式为“hh:mm”的两个字符串并返回格式相同的字符串。其中一个字符串是由用户提供的，我们减去在给定截止日期前提供服务所需的时间。

这是我用来跟踪值的带有 console.log() 语句的代码:

function subtractMinutes(time, minsToSubtract) {
    /* Converts "hh:mm" format to a total in minutes */
    function toMinutes (hh) {
        //if in the recursion it will be an int instead of a string
        if (hh < 0) {
          return Math.abs(hh);
        }
        hh = hh.split(':');
        return (parseInt(hh[0], 10) * 60) + parseInt(hh[1], 10);
    }

    /* Converts total in minutes to "hh:mm" format */
    function toText (m) {
        var minutes = m % 60;
        var hours = Math.floor(m / 60);

        minutes = (minutes < 10 ? '0' : '') + minutes;
        hours = (hours < 10 ? '0' : '') + hours;

        return hours + ':' + minutes;
    }
    console.log('time = '+time); //tracking values
    console.log('minsToSubtract = '+minsToSubtract); //tracking values

    time = toMinutes(time);
    console.log('time toMinutes = '+time); //tracking values
    minsToSubtract = toMinutes(minsToSubtract);
    console.log('minsToSubtract toMinutes = '+minsToSubtract); //tracking values

    var diff = time - minsToSubtract;
    console.log('diff = '+diff); //tracking values

    //if in recursion it will have to account for 24h/day instead of going to negative values
    if (diff < 0) {
      subtractMinutes("24:00", diff);
    } 
    //end of recursion when diff>0 and the result may be returned
    else {
      console.log('diff = '+diff); //tracking values
      var result = toText(diff);
      console.log('result = '+result); //tracking values
      return result; //at the end this value is correct, after this point it becomes "undefined"
    }
}
var result = subtractMinutes("0:35", "01:00");
console.log(result);
console.log('---------------------');

非常欢迎任何建议，尽管不会考虑包/插件/库。

-----编辑-----

给定rlemon's answer我尝试了两种不同的方法来尝试对此进行排序，但没有成功:

    if (diff < 0) {
      subtractMinutes("24:00", diff);
    } 
    //removed the else part of the statement as suggested 
    console.log('diff = '+diff); //tracking values
    var result = toText(diff);
    console.log('result = '+result); //tracking values
    return result;
}

这不再返回 undefined，但是 toText() 函数在递归解析正确的值后被调用，因此尝试转换值一秒钟时间，返回类似 "0-1:0-25" 的内容。

所以我认为我已经理解了这个问题，我尝试以另一种方式解决这个问题并执行了以下操作:

    var result;
    if (diff < 0) {
      subtractMinutes("24:00", diff);
    } else {
      console.log('diff after if= '+diff); //"diff after if= 1415"
      result = toText(diff);
      console.log('result = '+result); //"result = 23:35" CORRECT VALUE
      return result; //tried with this one on and off, same problem
    }
    console.log('result before return = '+result); //"result before return = undefined"
    return result;
}

我想建议在我的函数的最后一个结果上放置一个断点，以查看分配给它的值以及取消定义该值的实际情况。

Answer 1

在第一个函数调用后没有返回，所以它返回'undefined'。
举个简单的例子:

function foo( bar ) {
  if( bar ) {
    foo(false); // okay.. so you hit this and jump out of the if statement.
    // now you think because the return is going to be triggered in the next
    // pass that it will be the end result, but it isn't.
  } else {
    return 1;
  }
  // no return, so return undefined
}
foo(true)

将“递归”if 语句中的 subtractMinutes("24:00", diff); 更改为 return subtractMinutes("24:00", diff); 应该解决这个问题，因为现在它不会在第一次通话时终止。

查看演示:http://jsfiddle.net/rlemon/2YgyJ/

作为旁注:现在您不需要在那里有“else”语句，您可以完全省略它，只需在 if 之后运行代码:

//if in recursion it will have to account for 24h/day instead of going to negative values
if (diff < 0) {
    return subtractMinutes("24:00", diff); // because of the return here, the code below is not hit until the if condition returns false
}
//end of recursion when diff>0 and the result may be returned
console.log('diff = ' + diff); //tracking values
var result = toText(diff);
console.log('result = ' + result); //tracking values
return result;