jQuery Deferred - Variable scope in deferred getJS

问题:

I am adding deferred getJSON calls to an array inside a for loop that reference local variables inside their success function. The problem I am having is that when the success function is called, the local variable is taking the value from the last iteration of the loop. See below example:

var calls = [];
var arr = ['a','b','c'];
for (var a in arr) {
    calls.push(
        $.getJSON(window.location, function() { alert(arr[a]); })
    );
}
$.when.apply($,calls);

jsFiddle: http://jsfiddle.net/Me5rV/

This results in three alerts with the value 'c', whereas I would like the values 'a', 'b', and 'c'. Is this possible?

EDIT: The below works, but I'm not entirely sure why this differs?

var calls = [];
var arr = ['a','b','c'];
for (var a in arr) {
    calls.push(
        $.getJSON(window.location, function(x) {
            alert(x);
       }(arr[a]))
    );
}
$.when.apply($,calls);

jsFiddle: http://jsfiddle.net/Me5rV/1/

回答1:

Review what a loop like this:

var a = [];

for( var i = 0; i < 3; ++i ) {
    a.push( function() {
        alert(i);
    });
}

actually does:

var a = [], i = 0;

a.push( function(){
    alert(i);
});

i++;

a.push( function() {
    alert(i);
});

i++;

a.push( function() {
    alert(i);
});

i++;

//condition isn't met, loop terminates

alert(i) //alerts 3 because i is 3 now.
         //That's why all the functions alert 3, because they all 
         //refer to this i and its value is 3

Now you could do this instead (repetition removed):

a.push( function(i){
    return function() {
         alert(i); //Refers to the i passed as argument to the outer function
                   //not the global one
                   //The local i has whatever value the global i had when it was passed
                   //as argument
    };
}(i));