Closing dropdown by clicking outside in Javascript (tutorial clarification)

问题

I have attempted to implement the method of opening and closing a drop-down using Javascript via this tutorial on w3schools.com. While the function to "show" the drop-down works, the one to close it does not. Furthermore, there is no explanation alongside this code to explain why it should work, making it difficult to debug.

/* When the user clicks on the button, 
toggle between hiding and showing the dropdown content */
function myFunction() {
    document.getElementById("myDropdown").classList.toggle("show");
}

// Close the dropdown menu if the user clicks outside of it
window.onclick = function(event) {
  if (!event.target.matches('.dropbtn')) {

    var dropdowns = document.getElementsByClassName("dropdown-content");
    var i;
    for (i = 0; i < dropdowns.length; i++) {
      var openDropdown = dropdowns[i];
      if (openDropdown.classList.contains('show')) {
        openDropdown.classList.remove('show');
      }
    }
  }
}

My questions are, therefore,

1) whether the code in the tutorial should work for the purpose of closing the drop-down. (ANSWERED)

2) Could someone please clarify how/why this should work, for the sake of clarity for myself and future newbies who make come across the same tutorial and issue? (UNANSWERED)

Edit (MY ATTEMPT):

HTML:

<div class="sharedown">     
    <p onclick="shareVis()" class="sharebtn">&nbsp Share</p>
    <div id="mySharedown" class="sharedown-content">
        <a href="#">Self</a>
        <p>User</p><input type="text" name="user-name" placeholder="Share to">
        <a href="#">Community</a>
    </div> 
</div>

JS:

function shareVis() {
    document.getElementById("mySharedown").className = "show";
}

window.onclick = function(event) {
    if (!event.target.matches('sharebtn')) {

        var sharedowns = document.getElementsByClassName("sharedown-content");
        var i;
        for (i = 0; i < sharedowns.length; i++) {
            var openSharedown = sharedowns[i];
            if (openSharedown.classList.contains('show')) {
                openSharedown.classList.remove('show');
            }
        }
    }   
}

CSS:

/* Share dropdown menu */

p.sharebtn {

    color: darkgrey;
    font-family:calibri;
    padding: 0px;
    margin: 0px;
    font-size: 12;
    border: none;
    cursor: pointer;
    display:    inline; 
}

/* Dropdown button on hover & focus */
p.sharebtn:hover, p.sharebtn:focus {
    color: grey;

}

/* The container <div> - needed to position the dropdown content */

.sharedown {
    position: relative;
    display:    inline-block;

}

/* Dropdown Content (Hidden by Default) */
.sharedown-content {
    display: none;
    position: absolute;
    background-color:   #f1f1f1;
    min-width:  100px;
    box-shadow: 0 2px 4px 1px #C4E3F5;
    z-index:1; /* place dropdown infront of everything else*/
}

.sharedown-content a { 
color: black;
padding: 5px 5px;
text-decoration: none;
display: block;
}

/* Show the dropdown menu (use JS to add this class to the .dropdown-
content container when the user clicks on the dropdown button) */

.show {display: block;
    position: absolute;
    background-color:   #f1f1f1;
    min-width:  100px;
    box-shadow: 0 2px 4px 1px #C4E3F5;
    opacity: 1;
    z-index:1;}

回答1:

The issue lies in shareVis function. Here

document.getElementById("mySharedown").className = "show";

you are replacing #mySharedown class name to show. Then in window.onclick

var sharedowns = document.getElementsByClassName("sharedown-content");

you are not getting any sharedowns as you already replaced the class name to show.

You can either add show class into classList

document.getElementById("mySharedown").classList.add("show");

or replace the class name with sharedown-content show

document.getElementById("mySharedown").className = "sharedown-content show";

Working solution below:

function shareVis() {
    //document.getElementById("mySharedown").className = "sharedown-content show";
    document.getElementById("mySharedown").classList.add("show");
}

window.onclick = function(event) {
    if (!event.target.matches('.sharebtn')) {

        var sharedowns = document.getElementsByClassName("sharedown-content");
        var i;
        for (i = 0; i < sharedowns.length; i++) {
            var openSharedown = sharedowns[i];
            if (openSharedown.classList.contains('show')) {
                openSharedown.classList.remove('show');
            }
        }
    }
}

document.getElementById("mySharedown").addEventListener('click',function(event){
    event.stopPropagation();
});

#mySharedown{
  display: none;
  border: 1px solid black;
}

#mySharedown.show {
  display: block;
}

<div class="sharedown">     
    <p onclick="shareVis()" class="sharebtn">&nbsp Share</p>
    <div id="mySharedown" class="sharedown-content">
        <a href="#">Self</a>
        <p>User</p><input type="text" name="user-name" placeholder="Share to">
        <a href="#">Community</a>
    </div> 
</div>

Update

To prevent the second click within #mySharedown from hiding #mySharedown, you should add another click event for #mySharedown and prevent it from bubbling up, like this

document.getElementById("mySharedown").addEventListener('click',function(event){
    event.stopPropagation();
});

Updates are included in the working solution

回答2:

The example is fully functional and should work. Copy the following code below:

   <!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
.dropbtn {
    background-color: #3498DB;
    color: white;
    padding: 16px;
    font-size: 16px;
    border: none;
    cursor: pointer;
}

.dropbtn:hover, .dropbtn:focus {
    background-color: #2980B9;
}

.dropdown {
    position: relative;
    display: inline-block;
}

.dropdown-content {
    display: none;
    position: absolute;
    background-color: #f1f1f1;
    min-width: 160px;
    overflow: auto;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
    z-index: 1;
}

.dropdown-content a {
    color: black;
    padding: 12px 16px;
    text-decoration: none;
    display: block;
}

.dropdown a:hover {background-color: #ddd}

.show {display:block;}
</style>
</head>
<body>

<h2>Clickable Dropdown</h2>
<p>Click on the button to open the dropdown menu.</p>

<div class="dropdown">
<button onclick="myFunction()" class="dropbtn">Dropdown</button>
  <div id="myDropdown" class="dropdown-content">
    <a href="#home">Home</a>
    <a href="#about">About</a>
    <a href="#contact">Contact</a>
  </div>
</div>

<script>
/* When the user clicks on the button, 
toggle between hiding and showing the dropdown content */
function myFunction() {
    document.getElementById("myDropdown").classList.toggle("show");
}

// Close the dropdown if the user clicks outside of it
window.onclick = function(event) {
  if (!event.target.matches('.dropbtn')) {

    var dropdowns = document.getElementsByClassName("dropdown-content");
    var i;
    for (i = 0; i < dropdowns.length; i++) {
      var openDropdown = dropdowns[i];
      if (openDropdown.classList.contains('show')) {
        openDropdown.classList.remove('show');
      }
    }
  }
}
</script>

</body>
</html>

来源：https://stackoverflow.com/questions/50465697/closing-dropdown-by-clicking-outside-in-javascript-tutorial-clarification

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javascript

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web