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Targeting data attribute & running function issue

I'm trying to target different elements on the page by it's data-product, and setting a rule that if the data-product is a certain number then to run the function. But at the moment the functions runs multiple times and i'm not sure why this is.
var targetProduct = document.querySelectorAll('.content[data-product]').forEach(function(el) {
if ('1'.indexOf(el.getAttribute('data-product')) > -1) {
addMessage();
} else if ('2'.indexOf(el.getAttribute('data-product')) > -1) {
addMessage();
}
});
function addMessage() {
document.querySelectorAll('.product').forEach(function(el) {
el.insertAdjacentHTML("afterend", "<p style='color: red'>Sale</p>");
});
}
<div class="content" data-product="1">
<p class='product'>Product 1</p>
</div>
<div class="content" data-product="2">
<p class='product'>Product 2</p>
</div>
<div class="content" data-product="3">
<p class='product'>Product 3</p>
</div>
<div class="content" data-product="4">
<p class='product'>Product 4</p>
</div>

Your addMessage() function is adding the Sale text to all elements through the .forEach method. You should instead pass to it the element that needs to have .insertAdjacentHTML done to it and then inside the function only update that one element.
Here is a working example. Note that I have combined the two if-statements into one using the logical OR (||) operator.
const targetProduct = document.querySelectorAll('.content[data-product]')
targetProduct.forEach(function(el) {
const attr = el.getAttribute('data-product')
if ('1'.indexOf(attr) > -1 || '2'.indexOf(attr) > -1) {
addMessage(el)
}
})
function addMessage(el) {
el.insertAdjacentHTML('afterend', "<p style='color: red'>Sale</p>")
}
<div class="content" data-product="1">
<p class="product">Product 1</p>
</div>
<div class="content" data-product="2">
<p class="product">Product 2</p>
</div>
<div class="content" data-product="3">
<p class="product">Product 3</p>
</div>
<div class="content" data-product="4">
<p class="product">Product 4</p>
</div>

If you want to add a label to particular item, you could use this. Does it solve your problem?
var targetProduct = document.querySelectorAll('.content[data-product]').forEach(function(el) {
if ('1'.indexOf(el.getAttribute('data-product')) > -1) {
addMessage('1');
} else if ('2'.indexOf(el.getAttribute('data-product')) > -1) {
addMessage('2');
}
});
function addMessage(id) {
document.querySelectorAll('.content[data-product="' + id + '"] .product').forEach(function(el) {
el.insertAdjacentHTML("afterend", "<p style='color: red'>Sale</p>");
});
}
<div class="content" data-product="1">
<p class='product'>Product 1</p>
</div>
<div class="content" data-product="2">
<p class='product'>Product 2</p>
</div>
<div class="content" data-product="3">
<p class='product'>Product 3</p>
</div>
<div class="content" data-product="4">
<p class='product'>Product 4</p>
</div>

How do I create a show/hide loop in jQuery?

Here is my HTML with 3 questions and 3 answers:
<div class="faq-carousel">
<div class="all-questions question1">
<h4>Question 1</h4>
</div>
<div class="all-questions question2">
<h4>Question 2</h4>
</div>
<div class="all-questions question3">
<h4>Question 3</h4>
</div>
<div class=" all-answers answer1">
<p>Answer 1</p>
</div>
<div class=" all-answers answer2">
<p>Answer 2</p>
</div>
<div class=" all-answers answer3">
<p>Answer 3</p>
</div>
Here is my jQuery that shows/hides the 3 questions and answers:
jQuery(document).ready(function () {
"use strict";
jQuery(".all-answers").hide();
jQuery(".answer1").show();
jQuery(".all-questions").removeClass("highlighted");
jQuery(".question1").addClass("highlighted");
var slideNumber = 1;
jQuery(".question1").click(function () {
jQuery(".all-answers").hide();
jQuery(".answer1").show();
jQuery(".all-questions").removeClass("highlighted");
jQuery(".question1").addClass("highlighted");
slideNumber = 1;
});
jQuery(".question2").click(function () {
jQuery(".all-answers").hide();
jQuery(".answer2").show();
jQuery(".all-questions").removeClass("highlighted");
jQuery(".question2").addClass("highlighted");
slideNumber = 2;
});
jQuery(".question3").click(function () {
jQuery(".all-answers").hide();
jQuery(".answer3").show();
jQuery(".all-questions").removeClass("highlighted");
jQuery(".question3").addClass("highlighted");
slideNumber = 3;
}); });
How can I change the jQuery so that I can add more Q and A's to the HMTL without having to add more jQuery?
Many thanks!

The process you're trying to achieve here is to 'DRY' up your code, in other words, Don't Repeat Yourself.
To achieve what you need you can use common classes on the questions and answers, then relate the two together by their indexes, something like this:
"use strict";
jQuery(document).ready(function($) {
$('.question').click(function() {
$('.question').removeClass('highlighted');
var index = $(this).addClass('highlighted').index();
$('.answer').hide().eq(index).show();
});
});
.answer { display: block; }
.answer ~ .answer { display: none; }
.highlighted { background-color: #CC0; }
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="faq-carousel">
<div class="question">
<h4>Question 1</h4>
</div>
<div class="question">
<h4>Question 2</h4>
</div>
<div class="question">
<h4>Question 3</h4>
</div>
<div class="answer">
<p>Answer 1</p>
</div>
<div class="answer">
<p>Answer 2</p>
</div>
<div class="answer">
<p>Answer 3</p>
</div>
</div>
Alternatively, if you want to explicitly link the elements together, due to HTML structure restrictions for example, then you can use data attributes to specify the relationships between elements:
"use strict";
jQuery(document).ready(function($) {
$('.question').click(function() {
$('.question').removeClass('highlighted');
var target = $(this).addClass('highlighted').data('target');
$('.answer').hide().filter(target).show();
});
});
.answer { display: block; }
.answer ~ .answer { display: none; }
.highlighted { background-color: #CC0; }
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="faq-carousel">
<div class="question" data-target="#answer-01">
<h4>Question 1</h4>
</div>
<div class="question" data-target="#answer-02">
<h4>Question 2</h4>
</div>
<div class="question" data-target="#answer-03">
<h4>Question 3</h4>
</div>
<div class="answer" id="answer-01">
<p>Answer 1</p>
</div>
<div class="answer" id="answer-02">
<p>Answer 2</p>
</div>
<div class="answer" id="answer-03">
<p>Answer 3</p>
</div>
</div>

Use a data attribute with the answer id.
Add the eventListener to all questions at once using jQuery(".all-questions").click
use jQuery('.answer'+jQuery(this).data('answer')).show(); to show current answer.
this will use current element.
use jQuery(this).addClass("highlighted"); to add the class to current element
To add the slide number, use slideNumber = jQuery(this).data('answer');
jQuery(document).ready(function() {
"use strict";
jQuery(".all-answers").hide();
jQuery(".answer1").show();
jQuery(".all-questions").removeClass("highlighted");
jQuery(".question1").addClass("highlighted");
var slideNumber = 1;
jQuery(".all-questions").click(function() {
jQuery(".all-answers").hide();
jQuery('.answer'+jQuery(this).data('answer')).show();
jQuery(".all-questions").removeClass("highlighted");
jQuery(this).addClass("highlighted");
slideNumber = jQuery(this).data('answer');
});
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="faq-carousel">
<div data-answer="1" class="all-questions question1">
<h4>Question 1</h4>
</div>
<div data-answer="2" class="all-questions question2">
<h4>Question 2</h4>
</div>
<div data-answer="3" class="all-questions question3">
<h4>Question 3</h4>
</div>
<div class=" all-answers answer1">
<p>Answer 1</p>
</div>
<div class=" all-answers answer2">
<p>Answer 2</p>
</div>
<div class=" all-answers answer3">
<p>Answer 3</p>
</div>

Give all questions and answer elements a data-number attribute with the correct number then use
$(".all-questions").click(function() {
$(".all-questions").hide();
var slideNumber = $(this).data("number");
$(".answer"+slideNumber).show();
$(".all-questions").removeClass("highlighted");
$(this).addClass("highlighted");
}

With no changes to the HTML, something like this should do it :
jQuery(function ($) {
"use strict";
$('.all-questions').on('click', function() {
$('.all-answers').hide().filter('.answer' + $(this).index()).show();
$('.all-questions').removeClass('highlighted').filter(this).addClass('highlighted');
});
$(".question1").trigger('click');
});

Try this one. Very simple. The trick I used is to associated every question class name with it's answers class name. For instance, if Question 1 class name is question1, its answer class name is question1_answer. After that let the magic happen (you can add your style however you want. Just copy/paste and run that code an see what it does.
<!DOCTYPE html>
<html>
<head>
<script src="https://code.jquery.com/jquery-2.2.4.min.js" integrity="sha256-BbhdlvQf/xTY9gja0Dq3HiwQF8LaCRTXxZKRutelT44=" crossorigin="anonymous"></script>
<script>
function myFunction(classname){
var answerClassName = "." + classname + '_answer';
$(answerClassName).show();
//Hide all other answers
var otherAnswers = document.body.getElementsByTagName("div");
var l = otherAnswers.length;
for(i=0 ; i<l ;i++){
if(otherAnswers[i].className == classname){
//do nothing
}else{
var otherAnswersClassName = "." + otherAnswers[i].className + '_answer';
jQuery(otherAnswersClassName).hide();
}
}
}
</script>
</head>
<body>
<div class="question1" onclick="myFunction(this.className)">
<h4>Question 1</h4>
</div>
<div class="question1_answer">
<p>Answer 1</p>
</div>
<div class="question2" onclick="myFunction(this.className)">
<h4>Question 2</h4>
</div>
<div class="question2_answer">
<p>Answer 2</p>
</div>
</body>
</html>

Traverse an array of divs by clicking

I have a page with 'previous' and 'next' buttons to traverse an array of divs.
Clicking next button will copy the next div indexed by the var 'click' into #place's innerHTML.
Clicking previous button will copy the previous div as you can imagine.
The problem is, when I click the button alternately(click previous and next and previous then next...), the div would remain as it is and will not change to the next or previous div, unless I click the same button twice.
A simple alert message would confirm the functions are executing when alternately clicked. What's going on? Thank you.
var clicks = 0;
var pageDivs = document.getElementsByClassName("plist");
var displayPanel = document.getElementById("place");
function g() {
if (clicks >= pageDivs.length) {
clicks = 0;
}
displayPanel.innerHTML = pageDivs[clicks].innerHTML;
clicks += 1;
}
function f() {
clicks -= 1;
if (clicks < 0) {
clicks = pageDivs.length - 1;
}
displayPanel.innerHTML = pageDivs[clicks].innerHTML;
}
.plist {
display: none;
}
#place {
background-color: lightskyblue;
min-height: 5%;
min-width: 10%;
}
<div id="place">
<p></p>
</div>
<div class="plist">
<p>Testing 1</p>
</div>
<div class="plist">
<p>Testing 2</p>
</div>
<div class="plist">
<p>Testing 3</p>
</div>
<div class="plist">
<p>Testing 4</p>
</div>
<div class="plist">
<p>Testing 5</p>
</div>
<button type="button" onclick="f()" style="width: 10%;">Previous</button>
<button type="button" onclick="g()" style="width: 10%;">Next</button>

You need to consistently add and subtract in the same place.
But no need for two functions
NOTE: for an additional improvement, just hide and show instead of inserting code into the DOM
var clicks = -1, // necessary for initial next clicking
pageDivs = document.getElementsByClassName("plist"),
displayPanel = document.getElementById("place"),
len = pageDivs.length;
function copyIt(direction) {
clicks += direction;
if (clicks >= len) clicks = 0;
else if (clicks < 0) clicks = len - 1;
displayPanel.innerHTML = pageDivs[clicks].innerHTML;
}
.plist {
display: none;
}
#place {
background-color: lightskyblue;
min-height: 5%;
min-width: 10%;
}
<div id="place">
<p></p>
</div>
<div class="plist">
<p>Testing 1</p>
</div>
<div class="plist">
<p>Testing 2</p>
</div>
<div class="plist">
<p>Testing 3</p>
</div>
<div class="plist">
<p>Testing 4</p>
</div>
<div class="plist">
<p>Testing 5</p>
</div>
<button type="button" onclick="copyIt(-1)" style="width: 10%;">Previous</button>
<button type="button" onclick="copyIt(1)" style="width: 10%;">Next</button>

When you click Next, increment your counter first. You're doing it after, which is causing the issue.
Define your function f like this:
function g() {
clicks += 1; // increment at the start
...
}
var clicks = 0;
var pageDivs = document.getElementsByClassName("plist");
var displayPanel = document.getElementById("place");
function g() {
clicks += 1;
if (clicks >= pageDivs.length) {
clicks = 0;
}
displayPanel.innerHTML = pageDivs[clicks].innerHTML;
}
function f() {
clicks -= 1;
if (clicks < 0) {
clicks = pageDivs.length - 1;
}
displayPanel.innerHTML = pageDivs[clicks].innerHTML;
}
.plist {
display: none;
}
#place {
background-color: lightskyblue;
min-height: 5%;
min-width: 10%;
}
<div id="place">
<p></p>
</div>
<div class="plist">
<p>Testing 1</p>
</div>
<div class="plist">
<p>Testing 2</p>
</div>
<div class="plist">
<p>Testing 3</p>
</div>
<div class="plist">
<p>Testing 4</p>
</div>
<div class="plist">
<p>Testing 5</p>
</div>
<button type="button" onclick="f()" style="width: 10%;">Previous</button>
<button type="button" onclick="g()" style="width: 10%;">Next</button>

The problem was clicks counter was getting updated after updating the innerHTML.
Moving that to top fixed the issue.
$(document).ready(function() {
var clicks = -1;
var pageDivs = document.getElementsByClassName("plist");
var displayPanel = document.getElementById("place");
$("#prev").on("click", f);
$("#next").on("click", g);
function g() {
clicks += 1;
if (clicks >= pageDivs.length) {
clicks = 0;
}
displayPanel.innerHTML = pageDivs[clicks].innerHTML;
}
function f() {
clicks -= 1;
if (clicks < 0) {
clicks = pageDivs.length - 1;
}
displayPanel.innerHTML = pageDivs[clicks].innerHTML;
}
});
.plist {
display: none;
}
#place {
background-color: lightskyblue;
min-height: 5%;
min-width: 10%;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="place">
<p></p>
</div>
<div class="plist">
<p>Testing 1</p>
</div>
<div class="plist">
<p>Testing 2</p>
</div>
<div class="plist">
<p>Testing 3</p>
</div>
<div class="plist">
<p>Testing 4</p>
</div>
<div class="plist">
<p>Testing 5</p>
</div>
<button id="prev" type="button" style="width: 10%;">Previous</button>
<button id="next" type="button" style="width: 10%;">Next</button>

document.getElementById works but document.getElementsByTagName does not work [duplicate]

This question already has answers here:
What do querySelectorAll and getElementsBy* methods return?
(12 answers)
Closed 6 years ago.
To select all the p tag inside div - it works with document.getElementById() but not with document.getElementsByTagName(). Why is that?
<div id="myDIV">
<h2 class="example">A heading with class="example" in div</h2>
<p>Para 1 first</p>
<p>Para 2</p>
<p>Para 3</p>
<p>Para 4</p>
</div>
<button onclick="myFunction()">Submit</button>
This works
function myFunction() {
var x = document.getElementById("myDIV").querySelectorAll("p");
var i;
for (i = 0; i < x.length; i++) {
x[i].style.backgroundColor = "red";
}
}
This does not
function myFunction() {
var x = document.getElementsByTagName("div").querySelectorAll("p");
var i;
for (i = 0; i < x.length; i++) {
x[i].style.backgroundColor = "red";
}
}

I suggest the use of direct selector like :
document.querySelectorAll("#myDIV p");
//Or
document.querySelectorAll("div p");
Hope this helps.
var x = document.querySelectorAll("div p");
var i;
for (i = 0; i < x.length; i++) {
x[i].style.backgroundColor = "red";
}
<div id="myDIV">
<h2 class="example">A heading with class="example" in div</h2>
<p>Para 1 first</p>
<p>Para 2</p>
<p>Para 3</p>
<p>Para 4</p>
</div>

can you try this .
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
</head>
<body>
<div id="myDIV">
<h2 class="example">A heading with class="example" in div</h2>
<p>Para 1 first</p>
<p>Para 2</p>
<p>Para 3</p>
<p>Para 4</p>
</div>
<button onclick="myFunction()">Submit</button>
<script>
function myFunction() {
var x = document.querySelectorAll("p");
var i;
for (i = 0; i < x.length; i++) {
x[i].style.backgroundColor = "red";
}
}
</script>
</body>
</html>

Show and hide images with next previous button using Javascript

I have four divs, each div contain images and contents. one next and previous button. at a time only div will show. on click of next the second div have to be shown and first will hide. same upto fourth div. Need to use pure Javascript. No jquery or javascript plugins is allowed.... :(. Can anyone help me to do this?.
Thanks!
My html Code:
<html >
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>
<body>
<div id="container">
<div id="theProduct">
<div id="slides"> <img src="img/prev.jpg" width="29" height="29" alt="Arrow Prev"> <img src="img/next.jpg" width="29" height="29" alt="Arrow Next">
<!-- Show/hide Div strating here -->
<div class="slides_container">
<div class="div1">
<div class="img1">
<div class="price">
<ul>
<li class="strike">Rs. 1300</li>
<li class="offer">Rs. 1050</li>
</ul>
<div class="buy">Buy Now</div>
</div>
</div>
</div>
<div class="div2">
<div class="img2">
<div class="price">
<ul>
<li class="strike">Rs. 1300</li>
<li class="offer">Rs. 1050</li>
</ul>
<div class="buy">Buy Now</div>
</div>
</div>
</div>
<div class="div3">
<div class="img3">
<div class="price">
<ul>
<li class="strike">Rs. 1300</li>
<li class="offer">Rs. 1050</li>
</ul>
<div class="buy">Buy Now</div>
</div>
</div>
</div>
<div class="div4">
<div class="img4">
<div class="price">
<ul>
<li class="strike">Rs. 1300</li>
<li class="offer">Rs. 1050</li>
</ul>
<div class="buy">Buy Now</div>
</div>
</div>
</div>
</div>
<!-- Show/Hide Div ending here -->
</div>
<img src="img/example-frame.png" width="739" height="341" alt="Example Frame" id="frame"> </div>
</div>
</body>
</html>

Practical with this below
<style type="text/css" media="screen">
#container{position:relative;width:120px;height:120px;}
#container div{position:absolute;width:120px;height:120px;}
#box-red{background:red;}
#box-yellow{background:yellow;display:none;}
#box-green{background:green;display:none;}
#box-maroon{background:maroon;display:none;}
</style>
Javascipt
<script type="text/javascript">
var $c =0;
function next(){
var boxes = document.getElementsByClassName("box");
$c +=1;
if($c >= boxes.length) $c = 0;
for (var $i=0;$i<boxes.length;$i++){
boxes[$i].style.display = "none";
}
boxes[$c].style.display = "block";
return false;
}
function prev(){
var boxes = document.getElementsByClassName("box");
$c -=1;
if($c < 0) $c = (boxes.length-1);
for (var $i=0;$i<boxes.length;$i++){
boxes[$i].style.display = "none";
}
boxes[$c].style.display = "block";
return false;
}
</script>
HTML
<div id="container">
<div id="box-red" class="box">DIV1</div>
<div id="box-yellow" class="box">DIV2</div>
<div id="box-green" class="box">DIV3</div>
<div id="box-maroon" class="box">DIV4</div>
</div>
previous next

I think this is what you are loking for:
http://girlswhogeek.com/tutorials/2007/show-and-hide-elements-with-javascript
But you could put that in one function:
<script language="JavaScript">
function ShowHide(divId){
if(document.getElementById(divId).style.display == 'none')
{
document.getElementById(divId).style.display='block';
}
else
{
document.getElementById(divId).style.display = 'none';
}
}
</script>

function toggleImage(forwrad){
var imageConainer = document.getElementByID(Your Parent div id);
var images = imageContainer.childNodes;
var showIndex = '';
for(var i=0; i<images.length;i++){
if(images[i].style.display == 'block'){
images[i].style.display == 'none';
if(forwrad){
showIndex = 1+1;
}
else{
showIndex = 1-1;
}
}
}
images[showIndex].style.display = true;
}

technically it's possible with pure CSS as well. with the :target pseudo class. but that would pollute your url with # tags. But you may actually want that.
Also Selectivizr is a good polyfill for old browsers