JavaScript：從一組對象中獲取唯一值和它們的計數？

Question

使用jQuery，我如何遍歷一個對象，並獲得一個密鑰的唯一值與每個值的計數？

例如，對於此陣：

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 }, 
]; 

我希望得到以下：

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

我這有，這工作，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

你可以使用map用於此目的爲：

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

或

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

而且我可以向你指出的是，雖然這個代碼良好的外觀和緊湊的，這不是一般的快。最好讓你的代碼更更快，更容易看的：

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