如何在匿名函数调用中获取外部循环索引？

Question

I have the following javascript code :

var Person = [['John', 0, 0, 0],['Chris', 1, 0, 0]];
for (i = 0; i < Person.length; i++ )
{
    someObj.myMethod(Person[i][0], function (object) {
        console.log(i); //this prints 2, I want 0 and 1 as per the loop

        //here I want to access other members of Person[i] array, like Person[i][1], Person[i][2] and Person[i][3]
        //but these console.log() print 'undefined' because i = 2 !!
        console.log(Person[i][1]);
        console.log(Person[i][2]);
        console.log(Person[i][3]);
    });
}

Inside the anonymous function called inside my myMethod(), value of i is '2'. Please suggest how to get i = 0 in first cycle of for loop and then 1 in the second loop.

Answer 1

With a closure, this a solution:

var Person = [['John', 0, 0, 0],['Chris', 1, 0, 0]];
for (x = 0; x < Person.length; x++ )
{
    (function(i){   //We add a closure
        someObj.myMethod(Person[i][0], function (object) {
          console.log(i);
          console.log(Person[i][1]);
          console.log(Person[i][2]);
          console.log(Person[i][3]);
      });
    })(x);      //Call the closure function with the value of the counter
}

I changed the original counter to x to make it more understandable (so you dont confuse that variable with the original i ), but if it keeps named also i , it will work, too.

This way, each loop cycle has it's own variable x ( not shared ), so it doesn't get overwritten by the for loop, which was causing the problem ( i was shared) :)

Cheers

Answer 2

You have a leaky closure . Try this:

var Person = [['John', 0, 0, 0],['Chris', 1, 0, 0]];
for (i = 0; i < Person.length; i++ )
{
    doIt(i);
}

function doIt(i) {
    someObj.myMethod(Person[i][0], function (object) {
        console.log(i); //this prints 2, I want 0 and 1 as per the loop

        //here I want to access other members of Person[i] array, like Person[i][1], Person[i][2] and Person[i][3]
        //but these console.log() print 'undefined' because i = 2 !!
        console.log(Person[i][1]);
        console.log(Person[i][2]);
        console.log(Person[i][3]);
    });
}

Basically, your original inner anonymous function is being passed elsewhere to be executed later, by which stage the i variable has already been incremented to 2 in the for loop (they're effectively referencing the one copy of i ). Javascript is function scoped, so introducing a new function with it's own parameter to trap the specific value of i , you can decouple your anonymous function from the shared counter in the for loop.

Note that you can also make the function ( doIt in my example) an immediately executing anonymous function like Edgar has , which means that no one else can call the logically private closure function (though it arguably makes it a little harder to read to someone not used to javascript closures).

Answer 3

It depends entirely on when the callbacks are executed. If they are executed immediately , as the loop is running, then the values will be as you expect. The following outputs 0 and 1 :

var someObj = {
    myMethod: function(person, callback) {
        callback();
    }
}

var Person = [['John', 0, 0, 0],['Chris', 1, 0, 0]];
for (i = 0; i < Person.length; i++ )
{
    someObj.myMethod(Person[i][0], function (object) {
        console.log(i); 
    });
}

However, if the callbacks are stored and executed later, then (and this is the key thing to understand) each callback stored has closed over the same variable i and that variable's value is 2 after the loop has completed.

In other words, the closure is over the variable itself, not its value at the time the closure is created . As others have stated, this problem is easily solved using a wrapper function that receives i as an argument. This creates a new variable for each callback to close over.

Here's a contrived example:

var someObj = {

    callbacks: [],

    myMethod: function(person, callback) {
        someObj.callbacks.push(callback);
    }
}

var Person = [['John', 0, 0, 0],['Chris', 1, 0, 0]];
for (i = 0; i < Person.length; i++ )
{
    (function(i) {
        someObj.myMethod(Person[i][0], function (object) {
            console.log(i); 
        });
    })(i);
}

someObj.callbacks[0]();
someObj.callbacks[1]();

Another option in your case is to pass the elements of Person to your callback, instead of their index in the array.

Answer 4

Another way to deal with closure issue that you have is to create local variable and pass it to your function.

var Person = [['John', 0, 0, 0],['Chris', 1, 0, 0]];

for (var i = 0; i < Person.length; i++ ) {
    var x = i;

    someObj.myMethod(Person[x][0], function (object) {
        console.log(Person[x][1]);
        console.log(Person[x][2]);
        console.log(Person[x][3]);
    });
}