javascript - 如何将查询响应分配给 global.arrays，而不是 global.arrays 数组中的值？

Question

问题:

我正在尝试将对我的 MongoDB 数据库的查询结果有效地分配给全局数组。我基本上尝试将对全局数组的引用存储在一个数组中，以便我可以将 for 循环中的查询结果分配给所有这些引用。

这似乎是不可能的。你有什么建议？

---

代码:

    var arrays = [global.array1, global.array2, global.array3];

    var colsArray = ["array1","array2","array3"];

    var promises = colsArray.map(col => global.fetchCollection(col));

    Promise.all(promises).then(responses => {

        for (var d = 0; d < responses.length; d++) {
            arrays[d] = responses[d];
        } 

        console.log("VALUE INSIDE ARRAY of global.arrays: "+arrays[0]);
        console.log("VALUE OF global.array is still : "+global.array1);

    })

---

输出:

VALUE INSIDE ARRAY of global.arrays:: [object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
VALUE OF global.array is still : undefined

---

注意:

这当然可行。但当然不尽如人意:

        var arrays = [global.array1, global.array2, global.array3];

        var colsArray = ["array1","array2","array3"];

        var promises = colsArray.map(col => global.fetchCollection(col));

        Promise.all(promises).then(responses => {

                global.array1 = responses[0];
                global.array2 = responses[2];
                global.array3 = responses[3];

        })

---

编辑:

这不起作用:

    var arrays = [global.array1, global.array2, global.array3];

    var colsArray = ["array1","array2","array3"];

    var promises = colsArray.map(col => global.fetchCollection(col));

    Promise.all(promises).then(responses => {

        for (var d = 0; d < responses.length; d++) {
            arrays[colsArray[d]] = responses[d];
        } 

        console.log("VALUE INSIDE ARRAY of global.arrays: "+arrays[0]);
        console.log("VALUE OF global.array is still : "+global.array1);

    })

Answer 1

当你这样做时:

var arrays = [global.array1, global.array2, global.array3];

您将global.array1 等中的值复制 到数组 中。该值称为对象引用。它告诉 JavaScript 引擎数组在内存中的其他位置。 arrays[0] 和 global.array1 之间没有持续的链接(例如)；它们恰好包含相同的值。

您的代码替换数组中的值。这对 global.array1 等完全没有影响。

如果您确实有一个名为 global 的对象，那么您可以使用方括号符号(请参阅 this question's answers)对其进行索引以实际更改 global.array1 的值像这样:

for (var d = 0; d < responses.length; d++) {
    global["array" + (d + 1)] = responses[d];
} 

实例:

var global = {
    fetchCollection(col) {
        return new Promise(resolve => {
            setTimeout(() => {
                resolve("Response for " + col);
            }, Math.floor(Math.random() * 800));
        });
    }
};

var colsArray = ["array1", "array2", "array3"];

var promises = colsArray.map(col => global.fetchCollection(col));

Promise.all(promises).then(responses => {
    for (var d = 0; d < responses.length; d++) {
        global["array" + (d + 1)] = responses[d];
    } 

    console.log("VALUE OF global.array1 is : " + global.array1);

});

或者如果名称不是那样的数字，那么您将使用 colsArray 来获取名称:

for (var d = 0; d < responses.length; d++) {
    global[colsArray[d]] = responses[d];
} 

实例:

var global = {
    fetchCollection(col) {
        return new Promise(resolve => {
            setTimeout(() => {
                resolve("Response for " + col);
            }, Math.floor(Math.random() * 800));
        });
    }
};

var colsArray = ["array1", "array2", "array3"];

var promises = colsArray.map(col => global.fetchCollection(col));

Promise.all(promises).then(responses => {
    for (var d = 0; d < responses.length; d++) {
        global[colsArray[d]] = responses[d];
    } 

    console.log("VALUE OF global.array1 is : " + global.array1);

});

但我强烈建议您根本不要使用全局变量。