Why is my click event called twice in jquery?

问题:

Why is my click event fired twice in jquery?

HTML

<ul class=submenu>
    <li><label for=toggle><input id=toggle type=checkbox checked>Show</label></li>
</ul>

Javascript

$("ul.submenu li:contains('Show')").on("click", function(e) {
    console.log("toggle");
    if ($(this).find("[type=checkbox]").is(":checked")) console.log("Show");
    else console.log("Hide");
});

This is what I get in console:

toggle                     menu.js:39
Show                       menu.js:40
toggle                     menu.js:39
Hide                       menu.js:41


> $("ul.submenu li:contains('Show')")
[<li>​                                            ]
    <label for=​"toggle">​
      <input id=​"toggle" type=​"checkbox" checked>​
      "Show"
    </label>​
</li>​

回答1:

If I remember correctly, I've seen this behavior on at least some browsers, where clicking the label both triggers a click on the label and on the input.

So if you ignore the events where e.target.tagName is "LABEL", you'll just get the one event. At least, that's what I get in my tests:

• Example with both events | Source
• Example filtering out the e.target.tagName = "LABEL" ones | Source

回答2:

This behavior occurs when the input tag is structured within the label tag:

<label for="toggle"><input id="toggle" type="checkbox" checked>Show</label>

If the input checkbox is placed outside label, with the use of the id and for attributes, the multiple firing of the click event will not occur:

<label for="toggle">Show</label>
<input id="toggle" type="checkbox" checked>

回答3:

I recommend you use the change event on the input[type="checkbox"] which will only be triggered once. So as a solution to the above problem you might do the following:

$("#toggle").on("change", function(e) {
  if ($(this).is(":checked"))
    console.log("toggle: Show");
  else
    console.log("toggle: Hide");
});

https://jsfiddle.net/ssrboq3w/

The vanilla JS version using querySelector which isn't compatible with older versions of IE:

document.querySelector('#toggle').addEventListener('change',function(){
  if(this.checked)
    console.log('toggle: Show');
  else
    console.log('toggle: Hide');
});

https://jsfiddle.net/rp6vsyh6/