Access outside variable in loop from Javascript closure [duplicate]

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);
  });
});

The other way to approach this is to make sure the = items[i] business is effectively done by calling a function. In shorthand, this:

for (var i in this.items) {
    (function(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);
        });
    })(this.items[i]);
}

The anonymous function there is a bit messy, making it rather preferable to have a not-so-anonymous one around for the purpose, but it does the trick.

Javascript closures store references to their variables, so all of your onclick handlers are using the same variable.

You need to capture the variable in an intermediate function, like this:

function buildClickHandler(pageNumber) {
    return function()  {    //Create and return a new function
        alert(item.id);
        self.switchto(item.id);
    }
}

Then, use that function to create click handlers, like this:

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(buildClickHandler(item));
}

Each call to buildClickHandler creates a separate closure that has its own variable.

Access outside variable in loop from Javascript closure [duplicate]

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