如何使用 Javascript 读取 JSON 数组以获取值（如果存在）

Question

这是我的 JSON 代码。我将这个 json 存储在一个数组中。

{
"kind": "urlshortener#url",
"id": "http://goo.gl/2FIrtF",
"longUrl": "http://hike.com/?utm_source=facebook",
"status": "OK",
"created": "2015-09-22T13:45:53.645+00:00",
"analytics": {
    "allTime": {
        "shortUrlClicks": "1",
        "longUrlClicks": "1",
        "referrers": [
            {
                "count": "1",
                "id": "unknown"
            }
        ],
        "countries": [
            {
                "count": "1",
                "id": "IN"
            }
        ],
        "browsers": [
            {
                "count": "1",
                "id": "Chrome"
            }
        ],
        "platforms": [
            {
                "count": "1",
                "id": "Macintosh"
            }
        ]
    },
    "month": {
        "shortUrlClicks": "1",
        "longUrlClicks": "1",
        "referrers": [
            {
                "count": "1",
                "id": "unknown"
            }
        ],
        "countries": [
            {
                "count": "1",
                "id": "IN"
            }
        ],
        "browsers": [
            {
                "count": "1",
                "id": "Chrome"
            }
        ],
        "platforms": [
            {
                "count": "1",
                "id": "Macintosh"
            }
        ]
    },
    "week": {
        "shortUrlClicks": "1",
        "longUrlClicks": "1",
        "referrers": [
            {
                "count": "1",
                "id": "unknown"
            }
        ],
        "countries": [
            {
                "count": "1",
                "id": "IN"
            }
        ],
        "browsers": [
            {
                "count": "1",
                "id": "Chrome"
            }
        ],
        "platforms": [
            {
                "count": "1",
                "id": "Macintosh"
            }
        ]
    },
    "day": {
        "shortUrlClicks": "0",
        "longUrlClicks": "0"
    },
    "twoHours": {
        "shortUrlClicks": "0",
        "longUrlClicks": "0"
    }
},
"result": {
    "kind": "urlshortener#url",
    "id": "http://goo.gl/2FIuvF",
    "longUrl": "http://hike.com/?utm_source=facebook",
    "status": "OK",
    "created": "2015-09-22T13:45:53.645+00:00",
    "analytics": {
        "allTime": {
            "shortUrlClicks": "1",
            "longUrlClicks": "1",
            "referrers": [
                {
                    "count": "1",
                    "id": "unknown"
                }
            ],
            "countries": [
                {
                    "count": "1",
                    "id": "IN"
                }
            ],
            "browsers": [
                {
                    "count": "1",
                    "id": "Chrome"
                }
            ],
            "platforms": [
                {
                    "count": "1",
                    "id": "Macintosh"
                }
            ]
        },
        "month": {
            "shortUrlClicks": "1",
            "longUrlClicks": "1",
            "referrers": [
                {
                    "count": "1",
                    "id": "unknown"
                }
            ],
            "countries": [
                {
                    "count": "1",
                    "id": "IN"
                }
            ],
            "browsers": [
                {
                    "count": "1",
                    "id": "Chrome"
                }
            ],
            "platforms": [
                {
                    "count": "1",
                    "id": "Macintosh"
                }
            ]
        },
        "week": {
            "shortUrlClicks": "1",
            "longUrlClicks": "1",
            "referrers": [
                {
                    "count": "1",
                    "id": "unknown"
                }
            ],
            "countries": [
                {
                    "count": "1",
                    "id": "IN"
                }
            ],
            "browsers": [
                {
                    "count": "1",
                    "id": "Chrome"
                }
            ],
            "platforms": [
                {
                    "count": "1",
                    "id": "Macintosh"
                }
            ]
        },
        "day": {
            "shortUrlClicks": "0",
            "longUrlClicks": "0"
        },
        "twoHours": {
            "shortUrlClicks": "0",
            "longUrlClicks": "0"
        }
    }
}

}

在上面的 JSON 中，我们怎样才能得到分析 -> 天 -> 国家的存在？

我想先知道国家是否存在，如果不存在，显示一些价值。如果存在，它将尝试获取特定国家/地区的计数。

我从过去 5 个小时开始尝试这个，没有任何运气。

if(arr.analytics.day.countries !== undefined) {
         function thingscount(arr, platf) {
           var x = arr.analytics.day.countries.map(function(el) {
           return (platf.indexOf(el.id) != -1) ? parseInt(el.count) : 0; });
           var count = 0;
           for (var i = 0; i < x.length; i++) count += x[i];
           return count; 
           }       

        var one = thingscount(arr, ["US"]); 

        }else{
           var one = 0;

        }

如果当天有国家/地区，上面的代码可以正常工作，但有时，在我的 JSON 中将没有平台部分，在这种情况下它给了我

Uncaught TypeError: Cannot read property 'map' of undefined

我需要一种方法来检查平台是否存在，是否需要计数，是否没有为变量提供其他值。

更新：

我正在使用下面的代码来获取 IN 的计数。

当它有 IN 键和值时，它会给我结果。但是当它没有 IN 键时，它会显示“未定义计数”错误。

var month_inclicks = arr.analytics.month.countries.filter(function(el) { return el.id == "IN"; })[0].count;

如果我们要查找的键不存在，我们如何设置默认值？

Answer 1

虽然这不是 JSON，但我假设它是一个 javascript 对象。话虽如此，您需要考虑使用 hasOwnProperty 方法或 in 关键字。

例子：

if (arr.total.limited.hasOwnProperty('platforms')) { //do stuff

或

if ('platforms' in arr.total.limited) { //do something

Answer 2

我已更正您的 JSON。按照@CollinD 的建议使用hasOwnProperty

var arr = {
    total: {
        limited: {
            things: "451",
            platforms: [{
                count: "358",
                id: "Windows"
            }, {
                count: "44",
                id: "X11"
            }, {
                count: "42",
                id: "Macintosh"
            }, {
                count: "2",
                id: "Linux"
            }, {
                count: "1",
                id: "iPhone"
            }, {
                count: "1",
                id: "iPod"
            }]

        }
    }
};

Object.prototype.hasOwnProperty()

console.log(arr.total.limited.hasOwnProperty('platforms'));

DEMO

Answer 3

作为记录，您可以使用 reduce 将地图和“thingscount”函数中的计数滚动到一个操作中：

var getCount = function getCount( ary, find ) {
    return ary.reduce(function ( acc, record) {
        if (find.indexOf(record.id) !== -1) acc += parseInt(record.count, 10);
        return acc;
    }, 0);
};

内联用法：

if (arr.analytics.day.hasOwnProperty('countries')) {
    var find = ['IN'],
        count = arr.analytics.day.countries.reduce(function ( acc, record) {
            if (find.indexOf(record.id) !== -1) acc += parseInt(record.count, 10);
            return acc;
        }, 0);
}

或者用函数：

if (arr.analytics.day.hasOwnProperty('countries')) {
    var count = getCount(arr.analytics.day.countries, ['US','IN']);
}