JavaScript：從一組對象中獲取唯一值及其計數？

Question

使用jQuery，我如何迭代一個對象，並獲得具有每個值的計數的鍵的唯一值？

例如，對於此數組：

var electrons = [
    { name: 'Electron1', distance: 1 }, 
    { name: 'Electron2', distance: 1 }, 
    { name: 'Electron3', distance: 2 }, 
    { name: 'Electron4', distance: 2 }, 
    { name: 'Electron5', distance: 2 }, 
    { name: 'Electron6', distance: 2 }, 
    { name: 'Electron7', distance: 2 }, 
    { name: 'Electron8', distance: 2 }, 
    { name: 'Electron9', distance: 2 }, 
    { name: 'Electron10', distance: 2 }, 
    { name: 'Electron11', distance: 3 }, 
];

我想取回以下內容：

var distance_counts = {1: 2, 2: 8, 3: 1};

我有這個，它有效，但有點笨拙：

var radius_counts = {};
for (var i = 0; i < electrons.length; i++) { 
    if (electrons[i].distance in radius_counts) { 
         radius_counts[electrons[i].distance] += 1;
    } else { 
         radius_counts[electrons[i].distance] = 1;
    } 
}

Answer 1

您可以將地圖用於此目的：

var distances = {};
$.map(electrons,function(e,i) {
   distances[e.distance] = (distances[e.distance] || 0) + 1;
});

要么

var distances = {};
$.each(electrons,function(i,e) {
   distances[this.distance] = (distances[this.distance] || 0) + 1;
});

我也可以向你指出，雖然這個代碼看起來很好看並且緊湊，但這通常不會更快。 更好地使您的代碼更快，更容易看作：

var distances = {},e;
for (var i = 0,l=electrons.length; i < l; i++) { 
    e = electrons[i];
    distances[e.distance] = (distances[e.distance] || 0) + 1;
}