javascript - 如何找到数组中对象的总和

Question

我有这个对象数组，我想根据某些条件计算学生在特定科目上花费的时间。

我有这段代码来识别主题:但我很难找到这些特定主题的分钟总和。

$scope.total = 0;
$scope.found = [];

angular.forEach($scope.all, function(value, index) {
    angular.forEach(value.time, function (item, index) {

        $scope.total += value.time[index].minutes;

         if (item.subject === "english" || item.subject === "maths"    
         && $scope.total === 150) {
             $scope.found.push(value); 
         }
    });
});

"students": [
    {
        "id": 1,
        "name": "Natalie",
        "gender": "female",
        "time": [
            {
                "subject": "geography",
                "minutes": 100
            },{
                "subject": "english",
                "minutes": 20
            },{
                "subject": "maths",
                "minutes": 760
            }
        ]
    },{
        "id": 2,
        "name": "John",
        "gender": "male",
        "time": [
            {
                "subject": "spanish",
                "minutes": 450
            },{
                "subject": "maths",
                "minutes": 900
            },{
                "subject": "geography",
                "minutes": 200
            }
        ]
    }
]

Answer 1

这是使用 Javscript reduce 的一个很好的候选者和 filter功能。

Filter 将针对数组中的每个条目运行一个函数，返回一个新数组，其中仅包含该函数返回 true 的值。

Reduce 将在遍历数组中的每个条目时保留总值。您可以说它用于将数组“缩减”为单个值，这正是我们在这里要做的。

这是一个示例，假定 students 变量包含您的学生数组:

// We'll match against this variable
var desiredSubject = "maths"

// First, we iterate over each student in the array
var time = students.reduce(function(previousTime, student) {
  // Inside the array is another array, so we want to
  // reduce that one after filtering it to match our subject
  var subjectTime = student.time.filter(function(subject) {
    // Keep only subjects that match ours
    return subject.subject == desiredSubject;
  }).reduce(function(previousMinutes, subject) {
    // Then sum up the minutes from each one
    return previousMinutes + subject.minutes;
  }, 0); // Our initial sum is 0
  // Sum up each student's minutes
  return previousTime + subjectTime;
}, 0); // Again, with an initial sum of 0

请记住，reduce 函数的一个常见“陷阱”是您在 reduce 函数之后有起始值。很容易忘记把它放在那里。