jQuery.each（）未定义的问题

Question

I have the following code

function myFunction(items) {
   // this prints out 11
   alert(items.length);

   $(items).each(function(i, item) {
       // item is undefined for some reason
   }
}

It I alert the length of items, it has elements in it (11 to be exact). so how could 11 items be present, but jQuery still pass undefined?

Answer 1

The only explanation for this is the that items array contains values which are undefined, ie :

items = [undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined];

Both of the other answers are wholly incorrect. The first parameter of each is the index, not the value, and jQuery.fn.each calls jQuery.each. There is no disambiguation between them.

Answer 2

It sounds like you are not passing a jQuery wrappet set to your function. If you pass an array or an object you need to use the jQuery helper function $.each() like

$.each(items, function(index, element){
});

As I already mentioned several times in other answers, it is bad practice to loop over an array with .each() or javascripts native for..in .

If you are passing arrays use a standard for loop .

edit

As it turns out, you actually really can call the jQuery constructor with a standard array. But it seems like terrible karma to do so, you can't call 95% of all those jQuery methods, unless you want to crash / corrupt your code.

Answer 3

Since comments don't allow pretty code listings ... (do they?):

Animate will work against anything. It's documented as only working against CSS properties, but just to prove a point:

var foo = { x: 10, y: 12 }; 

$(foo).animate({ x: "+=20", y: "20" }, 1000, 
      function () { alert(foo.x + ":" + foo.y); }); 

Will spit out "30:20". Is this useful? Perhaps not in everyday work. :)

The trick with arrays is you'll set the same property across each entry. Example:

var foo = [{ x: 10 }, { x: 20 }]; 

$(foo).animate({ x: "+=30" }, 1000, 
      function () { alert(this.x); }); 

Spits out "40" and "50".