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

Question

QUESTION:

I am trying to assign to global arrays the results of queries to my MongoDB database efficiently. I essentially tried to store the references to the global arrays inside an array so that I could assign to all of them the results of the queries inside a for loop.

This does not seem to be possible. What would you suggest?

---

CODE:

    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);

    })

---

OUTPUT:

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

---

NB:

This would work of course. But quite unsatisfactory of course:

        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];

        })

---

EDIT:

This does not work:

    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

When you do this:

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

you copy the value in global.array1 etc. into arrays . That value is called an object reference. It tells the JavaScript engine where the array is, elsewhere in memory. There is no ongoing link between arrays[0] and global.array1 (for instance); they both just happen to contain the same value.

Your code replaces the values in arrays . This has no effect at all on global.array1 etc.

If you really have an object called global , then you can index into it with brackets notation (see this question's answers ) to actually change the value of global.array1 like this:

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

Live Example:

 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); });

Or if the names are not numeric like that, then you'd use your colsArray to get the name:

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

Live Example:

 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); });

But I'd strongly encourage you not to use globals at all.