为什么我将数组值设为“未定义”

Question

I'm a rookie in node.js so I apologize if my question is immature .Basically I'm trying to iterate thorough an array of values for performing certain operation but I don't know why I'm getting the values as undefined .

Code:

for (var i = 0; i < array.length; ++i) {
    console.log(array[i]); //At this point I'm getting values without any problem
    var sQ = {
        _tag: array[i],
        _pid: data.change_caption_post_mail,
        time: data.change_caption_post_time,
    };

    db.collection('tags')
        .find(sQ)
        .count({}, function(error, numOfDocs) {
            if (error) throw err;
            console.log(array[i]);
            //But here I'm getting values as **undefined**
        });
}

Answer 1

Replace

var i = 0; i < array.length; ++i

by

let i = 0; i < array.length; i++

and you're done.

for (let i = 0; i < array.length; i++) {
    console.log(array[i]); //At this point I'm getting values without any problem
    var sQ = {
        _tag: array[i],
        _pid: data.change_caption_post_mail,
        time: data.change_caption_post_time,
    };

    db.collection('tags')
        .find(sQ)
        .count({}, function(error, numOfDocs) {
            if (error) throw err;
            console.log(array[i]);
            // i will be array.length here in all your callbacks
        });
}

Cause of the problem: lack of understanding scope

Check this example to understand the problem:

var creates function scope

 var funcs = [] for (var i = 0; i < 10; i++) { funcs.push(function() { console.log(i) }) } funcs.forEach(function(func) { func() }) 

While you might expect this forEach loop to result in number 0 to 9 being printed, instead you get ten times 10 . The cause of this is the variable i being declared using var keyword, which creates a function scope that leads to each function in funcs holding a reference to the same i variable. At the time the forEach loop is executed, the previous for -loop has ended and i holds 10 (9++ from the last iteration).

Compare how ES6's let , which creates block scope instead of function scope , behaves in this regard:

let (ES6 or officially ES2015 ) creates block scope :

 var funcs = [] for (let i = 0; i < 10; i++) { funcs.push(function() { console.log(i) }) } funcs.forEach(function(func) { func() }) 

Because let creates block scope , each iteration of the for loop has its "own" variable i .

ES5 solution using an IIFE wrapper

If you need an ES5 solution, an IIFE ( i mmediately i nvoked f unction e xpression) wrapper would be the way to go:

 var funcs = [] for (var i = 0; i < 10; i++) { funcs.push((function(value) { return function() { console.log(value) } }(i))) } funcs.forEach(function(func) { func() }) 

Here, i is passed as a parameter to each function which stores its own copy value .

The same is true for for..in loops:

 var funcs = [], obj = { first: "first", last: "last", always: "always" } for (var key in obj) { funcs.push(function() { console.log(key) }) } funcs.forEach(function(func) { // outputs: "always", "always", "always" func() }) 

Again, all functions in funcs hold the reference to the same key because var key creates a function scope that lives outside of the for..in loop. And again, let produces the result you'd probably rather expect:

 var funcs = [], obj = { first: "first", last: "last", always: "always" } for (let key in obj) { funcs.push(function() { console.log(key) }) } funcs.forEach(function(func) { func() }) 

Also compare the excellent (!) book

Nicholas C. Zakas: "Understanding ES6" , no starch press, p. 8-9.

from which the examples were taken.

Answer 2

Change var i = 0 to let i = 0 .

for (let i = 0; i < array.length; ++i) {
console.log(array[i]); //At this point I'm getting values without any problem
var sQ = {
    _tag: array[i],
    _pid: data.change_caption_post_mail,
    time: data.change_caption_post_time,
};

db.collection('tags')
    .find(sQ)
    .count({}, function (error, numOfDocs) {
        if (error) throw err;
        console.log(array[i]);
    });

}

NB When the callback function ( function (error, numOfDocs) {...} ) is executing the for loop is finished. if we declare var i then i is equal to array.length when executing callback so, array[array.length] returns undefined. One of the solutions is to use ES6 let which will create callback's block scope i for each iteration.

Answer 3

The problem is that in your callback function, the reference of i remains only one which is already changed by the time callback is called.

you can change your code to

for (var i = 0; i < array.length; ++i) {
    (function(){
    var index = i;
    console.log(array[index]); //At this point I'm getting values without any problem
    var sQ = {
        _tag: array[index],
        _pid: data.change_caption_post_mail,
        time: data.change_caption_post_time,
    };

    db.collection('tags')
        .find(sQ)
        .count({}, function(error, numOfDocs) {
            if (error) throw err;
            console.log(array[index]);
            //But here I'm getting values as **undefined**
        });
    })();
}

it should work fine with this tweak