在Javascript中使用闭包时，为什么mouseover事件不起作用？

Question

I have the following code

<ul>
  <li>one</li>
  <li>two</li>        
  <li>three</li>
  <li>four</li>
</ul>

var lists = document.getElementsByTagName("li");

for(var i=0, len = lists.length; i< len; i++){
    lists[i].onmouseover = function(){
        console.log(i);
    }(i);
}

Expected result: when mouse over each li , I got 0 or 1 or 2 or 3 in the console, but I only got those number when refresh the page not in mouseover , anybody knows why?

Answer 1

The "calling parenthesis" (i) after the function expression execute the function immediately and assign its return value as event handler (which is undefined ). Here is an example with a function declaration, which makes it easier to see (hopefully):

function foo(i) {
    console.log(i);
}

// in the loop
lists[i].onmouseover = foo(i);

See how foo is called and the return value is assigned to lists[i].onmouseover ?

You have to return a function from the immediately invoked function expression:

lists[i].onmouseover = (function(i){
    return function() {
        console.log(i);
    };
}(i));

or with a function declaration:

function createHandler(i) {
    return function() {
        console.log(i);
    };
}

// in the loop
lists[i].onmouseover = createHandler(i);

More info: JavaScript closure inside loops – simple practical example