Access Outside Variable in Loop from JavaScript Closure

Access outside variable in loop from Javascript closure

The problem you have here is that the variable item changes with each loop. When you are referencing item at some later point, the last value it held is used. You can use a technique called a closure (essentially a function that returns a function) to quickly scope the variable differently.

    for (var i in this.items) {
            var item = this.items[i];
            $("#showcasenav").append("<li id=\"showcasebutton_"+item.id+"\"><img src=\"/images/showcase/icon-"+item.id+".png\" /></li>");
            $("#showcasebutton_"+item.id).click( 
                // create an anonymous function that will scope "item"
                (function(item) {
                   // that returns our function 
                   return function() {
                    alert(item.id);
                    self.switchto(item.id);
                   };
                })(item) // immediately call it with "item"
            );
    }

A side note - I see that you have jQuery here. It has a helper function $.each() that can be used with arrays, and can be a shortcut for simple for/each loops. Because of the way the scoping works in this call - you wont need to use a closure because "item" is already the parameter of the function when it was called, not stored in a var in the parent function's scope, like was true in your example.

$.each(this.items,function(i, item) {
  $("#showcasenav").append("<li id=\"showcasebutton_"+item.id+"\"><img src=\"/images/showcase/icon-"+item.id+".png\" /></li>");
  $("#showcasebutton_"+item.id).click(function() {
    alert(item.id);
    self.switchto(item.id);
  });
});

Accessing variables trapped by closure

Variables within a closure aren't directly accessible from the outside by any means. However, closures within that closure that have the variable in scope can access them, and if you make those closures accessible from the outside, it's almost as good.

Here's an example:

var A = function(b) {
    var c = b + 100;
    this.access_c = function(value) {
        // Function sets c if value is provided, but only returns c if no value
        // is provided
        if(arguments.length > 0)
            c = value;
        return c;
    };
    this.twain = function() {
        return 2 * c;
    };
};
var a_inst = new A(123);
var my_c = a_inst.access_c();
// my_c now contains 223
var my_2c = a_inst.twain();
// my_2c contains 446
a_inst.access_c(5);
// c in closure is now equal to 5
var newer_2c = a_inst.twain();
// newer_2c contains 10

Hopefully that's slightly useful to you...

Javascript closures with let variable

In the first code example, you have three different i variables and in each iteration of the loop, callback function of setTimeout closes over a different copy of i variable.

In the second code example, there is only one i variable and in each iteration of the loop, callback function of setTimeout closes over the same i variable.

There is only one i variable in the second code example because you have declared it outside the loop; as a result, each iteration of the loop sees the same variable i.

In order to get the same output in the second code example, change it as shown below:

let i = 0;

for (i = 0; i < 3; i++) {

  let j = i; // save a copy of the current value of 'i'
  
  setTimeout(() => {
    console.log(j);  // closure over a different 'j' variable in each iteration
  }, 1000);
}

Javascript closures - variable scope question

Its because at the time item.help is evaluated, the loop would have completed in its entirety. Instead, you can do this with a closure:

for (var i = 0; i < helpText.length; i++) {
   document.getElementById(helpText[i].id).onfocus = function(item) {
           return function() {showHelp(item.help);};
         }(helpText[i]);
}

JavaScript doesn't have block scope but it does have function-scope. By creating a closure, we are capturing the reference to helpText[i] permanently.

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