JavaScript:关于 JSON 数据操作的 2 个问题

Question

这个问题已经有答案了:

From an array of objects, extract value of a property as array (25 个回答)

已关闭 9 年前。

我有两个对象数组:

data = [
        {"value": 10.2, "cat": "A", "state": "MA"},
        {"value": 5.3, "cat": "B", "state": "MA"},
        {"value": 20.1, "cat": "A", "state": "NY"},
        {"value": 17.3, "cat": "B", "state": "NY"},
        {"value": 7.7, "cat": "A", "state": "CT"},
        {"value": 11.3, "cat": "B", "state": "CT"}
       ];

reference = [
             {"state": "MA", "increment": 4.1},
             {"state": "NY", "increment": 0.3},
             {"state": "CT", "increment": 20.6}
            ];

问题

1. 我怎样才能拉出所有state来自 reference 的字段没有循环？具体来说，像这样的函数 reference.pullElement("state")返回类似 ["MA", "NY", "CT"] 的内容。这存在吗？我是否必须创建一个 reference对象并将以下逻辑构建到方法中？

   var states = [];
for (var i = 0; i < reference.length; i++) {
states.push(reference[i]["state"]);
}

2. 如何添加 increment来自 reference 的字段至value领域 data ，这样我就可以有一个像这样的新数组。

   output = [
{"value": 14.3, "cat": "A", "state": "MA"},
{"value": 9.4, "cat": "B", "state": "MA"},
{"value": 20.4, "cat": "A", "state": "NY"},
{"value": 17.6, "cat": "B", "state": "NY"},
{"value": 28.3, "cat": "A", "state": "CT"},
{"value": 31.9, "cat": "B", "state": "CT"}
];

我正在尝试对问题 2 执行以下操作，但实际数据的类别比这多得多。这是最有效的方法吗？

var output = data.slice();
for (var i = 0; i < data.length; i++) {
    for (var j = 0; j < reference.length; j++) {
        if (data[i]["state"] == reference[j]["state"]) {
            output[i]["value"] = data[i]["value"] + reference[j]["increment"];
        }
    }
}

谢谢!

Answer 1

如果您能够使用数组上现在可用的(相对较新的)方法，则可以执行以下操作。

1.

// Produce an array of state strings
var states = reference.map(function(el) { return el.state; });

2.

// Produce an object with state strings as keys and increments as the values
var stateIncMap = reference.reduce(function(items, item) {
    items[item.state] = item.increment;
    return items;
}, {});

// Here's an alternate version of the above, using forEach() instead of reduce():
// var stateIncMap = {};
// reference.forEach(function(el) {
//     stateIncMap[el.state] = el.increment;
// });

// Update elements' values using the increments
data.forEach(function(el) {
    el.value += stateIncMap[el.state];
});

http://jsfiddle.net/vY5Sr/