JavaScript click handler not working as expected i

问题:

This question already has answers here:

How do JavaScript closures work? (86 answers)

Closed 6 years ago.

I'm trying to learn JS and got an issue.

I tried many things and googled but all in vain. Following piece of code doesn't work as expected. I should get value of i on click but it always returns 6. I'm pulling my hair out., please help.

for (var i = 1; i < 6; i++) {

    console.log(i);

    $("#div" + i).click(
        function() {
            alert(i);
        }
    );
}

jsfiddle

回答1:

Working DEMO

This is a classic JavaScript closure problem. Reference to the i object is being stored in the click handler closure, rather than the actual value of i.

Every single click handler will refer to the same object because there’s only one counter object which holds 6 so you get six on each click.

The workaround is to wrap this in an anonymous function and pass i as argument. Primitives are copied by value in function calls.

for(var i=1; i<6; i++) {
     (function (i) {
        $("#div" + i).click(
            function () { alert(i); }
        );
     })(i);
}

UPDATE

Updated DEMO

Or you can use 'let' instead var to declare i. let gives you fresh binding each time. It can only be used in ECMAScript 6 strict mode.

'use strict';

for(let i=1; i<6; i++) {

        $("#div" + i).click(
            function () { alert(i); }
        );
 }

回答2:

The problem is that as you iterate through the loop, i is incremented. It ends up with a value of 6. When you say alert(i) you are asking javascript to tell you what the value of i is at the time the link is clicked, which by that point is 6.

If you want to get the contents of the box instead you could do something like this:

for (var i = 1; i < 6; i++) {

    console.log(i);

    $("#div" + i).click(function(e) {
        alert($(this).text());
    });
}

Working example: http://jsfiddle.net/rmXcF/2/

回答3:

$("#div" + i).click(
    function() {
        alert(i);
    }
);

It's because it's using the value of i as a closure. i is remembered through a closure which increases at every stage of the foor loop.

$("#div" + i).click(function(event) {
    alert($(event.target).attr("id").replace(/div/g, ""));
});