检查数组是否包含具有特定属性值的对象 [英] Check if array contains an object with the value of a specific property

问题描述

我有一个大约30个对象的数组。每个对象都有一个名为registration的属性。注册属性是我用来唯一标识每个对象的属性。每个对象还包含一个时间戳（总是唯一的）。

I have an array of around 30 objects. Each object has a property called 'registration'. The registration property is what I use to uniquely identify each object. Each object also contains a timestamp (always unique).

每30秒我从API下载这30个对象的新实例。新对象可能会添加到当前数组中。我需要想出一种检查新对象是否在旧（当前）对象数组中的方法。如果对象不包含唯一的时间戳，那么这将非常简单，但由于每个实例都有所不同，这将无效。

Every 30 seconds I download a new instance of these 30 objects from an API. There is a possibility that a new object will be added to the current array. I need to come up with a way of checking if the new object is in the array of old (current) objects. If the objects didn't contain a unique timestamp then this would be very simple but as this varies for every instance this won't work.

到目前为止我所拥有的：

What I have so far:

newDownloadedArray = JSON.parse(newDownloadedArray);

for (var i = 0; i < currentArrayObjects.length; i++) {
    for (var j = 0; j < newDownloadedArray.length; j++) {

         /* 
         *  This is where I'm stuck.
         *  I now need to check if newDownloadedArray[j].registration is
         *  the value of any registration property inside the currentArrayObjects
         *  array.
         *
         *  If it is, then I know this is a new object.
         */

    }
}

推荐答案

这是一个解决方案：

var isNewObject = function(newObject) {
    return !currentArrayObjects.some(function(currentObject) {
        return newObject.registration == currentObject.registration;
    });
};
var onlyNewObjects = newDownloadedArray.filter(isNewObject);

我们基本上说对于中的每个对象> newDownloadedArray ，查看 currentArrayObjects 中的每个对象，直到找到匹配的注册。如果这样做，该对象包含在 onlyNewObjects 中。如果不包含，则不是。

We're basically saying "For every object in newDownloadedArray, look at every object in currentArrayObjects until you find one that has a matching registration. If you do, that object is included in onlyNewObjects. If you don't, it isn't.

注意 Array.prototype.filter 和 Array.prototype.some 仅在IE 9+中可用，因此您可能希望使用等效的辅助方法或实用程序库（如下划线）如果你想支持旧的浏览器。

Note that Array.prototype.filter and Array.prototype.some are only available in IE 9+, so you might want to use an equivalent helper method or utility library (like underscore) if you want to support older browsers.

这不是很有效。对于30个项目的阵列，我们的工作是最差的900吨imes（如果新数组是完全唯一的，因为它必须为每个 newDownloadedArray currentArrayObjects $ c>）。

This isn't very efficient. For arrays of 30 items, we're doing work a worst-case of 900 times (if the new array was completely unique, because it has to search through all 30 of the currentArrayObjects for each one of the newDownloadedArray).

但在浏览器方面，这并不是很多。你可以做很多事情来加速它。例如，我们可以构建一个包含所有注册的对象，而不是在谓词中搜索 currentArrayObjects ：

But that's not really a lot in browser terms. And you can do a lot to speed it up. For example, instead of searching through the currentArrayObjects in the predicate, we could build an object with all the registrations:

// We want a set of registrations, but JavaScript doesn't
// have a native set class, so we're going to use the keys 
// of an object to simulate sets, because object keys are
// basically sets of strings. Note that this won't work if
// registration isn't a string.
var currentRegistrations = {};
currentArrayObjects.forEach(function(currentObject) {
    // AKA currentRegistrationSet.add(currentObject.registration) if we
    // had an actual set class. I chose 'true' somewhat at random
    // because it felt right; we'll never actually be accessing
    // the value.
    currentRegistrations[currentObject.registration] = true;
});
var isNewObject = function(newObject) {
    // AKA !currentRegistrationSet.contains(newObject.registration) if we
    // had an actual set class.
    return !currentRegistrations.hasOwnProperty(newObject.registration);
}
var onlyNewObjects = newDownloadedArray.filter(isNewObject);

（同样需要注意 Array.prototype.forEach ）

(Same caveat about Array.prototype.forEach)

现在我们只需要做大约60次操作 - 30次提前构建对象，另外30次来检查每个操作。

Now we only have to do about 60 operations -- 30 to build the object ahead of time, and 30 more to check each one.

您的解决方案与我发布的第一个解决方案相差不远。但是您为循环切换了。它可能是：

Your solution wasn't far off from the first one I posted. But you switched the for loops. It could be:

newDownloadedArray = JSON.parse(newDownloadedArray);

var onlyNewObjects = []
for (var i = 0; i < newDownloadedArray.length; i++) {
    var isNewObject = true;
    for (var j = 0; j < currentArrayObjects.length; j++) {
        if (newDownloadedArray[i].registration == currentArrayObjects[j].registration) {
            isNewObject = false;

            break; // no reason to keep looking; we know it isn't new
        }
    }
    if (isNewObject) {
        onlyNewObjects.push(newDownloadedArray[i]);
    }
}

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