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javascript: how do i get the value of a specific attribute in a nested div

To simplify what I want to achieve, I want to get popular-laptop-deals from data-code="popular-laptop-deals" using a javascript bookmarklet alert.
<div class="display-table-column scroll-item display-block-xs top-padding-mini-xs tile-highlight category-tile" data-code="popular-laptop-deals" data-testid="tile-highlight">
These belong to a nested div where there are other <div> with data-code="". I'm trying to get the function to run through all and get all the other values from data-code="".
I have tried using the following script but it returns "undefined":
javascript:alert(document.getElementsByName("data-code")[0]);
Appreciate if someone could show or guide me on how i can achieve this.
Thanks in advance.

document.getElementsByName("data-code") is an empty NodeList because there are no elements with names, hence, no element with attribute data-code.
You can change getElementsByName to getElementsByTagName, etc. and then get its attribute value.
Also, data-* attributes can be accessed via the dataset property of an element, e.g. yourDiv.dataset.code.
alert(document.getElementsByTagName('div')[0].dataset.code);
<div class="display-table-column scroll-item display-block-xs top-padding-mini-xs tile-highlight category-tile" data-code="popular-laptop-deals" data-testid="tile-highlight">

With this, you can check all div tags on the page and add them to the array if contain data-code.
var divs = document.getElementsByTagName("div");
var result = [];
for (var i = 0; i < divs.length; i++) {
if (typeof divs[i].dataset.code !== "undefined") {
result.push(divs[i].dataset.code);
}
}
console.log(result)

With Jquery, you can achieve like as below.
Jquery:
alert($("div").attr("data-code"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="display-table-column scroll-item display-block-xs top-padding-mini-xs tile-highlight category-tile" data-code="popular-laptop-deals" data-testid="tile-highlight">

with querySelector select tag and with getAttribute select attribute
var attr_code = document.querySelector('div').getAttribute('data-code');
alert(attr_code);
<div class="display-table-column scroll-item display-block-xs top-padding-mini-xs tile-highlight category-tile" data-code="popular-laptop-deals" data-testid="tile-highlight">

Get all items that start with class name

I'm trying to only show certain divs. The way I have decided to do this is to first hide all elements that start with "page" and then only show the correct divs. Here's my (simplified) code:
<form>
<input type="text" onfocus="showfields(1);">
<input type="text" onfocus="showfields(2);">
</form>
<div class="page1 row">Some content</div>
<div class="page1 row">Some content</div>
<div class="page2 row">Some content</div>
<div class="page2 row">Some content</div>
<script>
function showfields(page){
//hide all items that have a class starting with page*
var patt1 = /^page/;
var items = document.getElementsByClassName(patt1);
console.log(items);
for(var i = 0; i < items.length; i++){
items[i].style.display = "none";
}
//now show all items that have class 'page'+page
var item = document.getElementsByClassName('page' + page);
item.style.display = '';
}
</script>
When I console.log(items); I get a blank array. I'm pretty sure the regexp is right (get all items starting with 'page').
The code I'm using is old school JS, but I'm not adverse to using jQuery. Also if there is a solution that doesn't use regexp, that's fine too as I'm new to using regexp's.

getElementsByClassName only matches on classes, not bits of classes. You can't pass a regular expression to it (well, you can, but it will be type converted to a string, which is unhelpful).
The best approach is to use multiple classes…
<div class="page page1">
i.e. This div is a page, it is also a page1.
Then you can simply document.getElementsByClassName('page').
Failing that, you can look to querySelector and a substring matching attribute selector:
document.querySelectorAll("[class^=page]")
… but that will only work if pageSomething is the first listed class name in the class attribute.
document.querySelectorAll("[class*=page]")
… but that will match class attributes which mention "page" and not just those with classes which start with "page" (i.e. it will match class="not-page".
That said, you could use the last approach and then loop over .classList to confirm if the element should match.
var potentials = document.querySelectorAll("[class*=page]");
console.log(potentials.length);
elementLoop:
for (var i = 0; i < potentials.length; i++) {
var potential = potentials[i];
console.log(potential);
classLoop:
for (var j = 0; j < potential.classList.length; j++) {
if (potential.classList[j].match(/^page/)) {
console.log("yes");
potential.style.background = "green";
continue elementLoop;
}
}
console.log("no");
potential.style.background = "red";
}
<div class="page">Yes</div>
<div class="notpage">No</div>
<div class="some page">Yes</div>
<div class="pageXXX">Yes</div>
<div class="page1">Yes</div>
<div class="some">Unmatched entirely</div>

Previous answers contain parts of the correct one, but none really gives it.
To do this, you need to combine two selectors in a single query, using the comma , separator.
The first part would be [class^="page"], which will find all the elements whose class attribute begins with page, this selector is thus not viable for elements with multiple classes, but this can be fixed by [class*=" page"] which will find all the elements whose class attribute have somewhere the string " page" (note the space at the beginning).
By combining both selectors, we have our classStartsWith selector:
document.querySelectorAll('[class^="page"],[class*=" page"]')
.forEach(el => el.style.backgroundColor = "green");
<div class="page">Yes</div>
<div class="notpage">No</div>
<div class="some page">Yes</div>
<div class="pageXXX">Yes</div>
<div class="page1">Yes</div>
<div class="some">Unmatched entirely</div>

You can use jQuery solution..
var $divs = $('div[class^="page"]');
This will get all the divs which start with classname page

$(document).ready(function () {
$("[class^=page]").show();
$("[class^=page]").hide();
});
Use this to show hide div's with specific css class it will show/hide all div's with css class mention.

Changing properties of all <hr> html elements

In css I have set all the <hr> elements in my html to "display:none;" which works.
I have an onclick event listener set up to change the "display" to "block".
I use:
document.getElementsByTagName("hr").innerHTML.style.display = "block";
I get an error "Cannot read property 'style' of undefined".

Do it the following way:
var hrItems = document.getElementsByTagName("hr");
for(var i = 0; i < hrItems.length; i++) {
hrItems[i].style.display = 'block';
}

This is incorrect in two ways
getElementsByTagName gives you a list on elements and there is no method to operate on all elements, so you'll have to loop through all of them and add the required style individually.
innerHTML returns a string containing the mark up in an element but <hr> doesn't have any thing in it and the style property is on the <hr> itself.
var hrs = document.getElementsByTagName("hr");
for(var i = 0; i < hrs.length; i++) {
hrs[i].style.display = 'block';
}

Simple (and very effective) solution:
tag your body with a class-element
<body class="no_hr"> <article><hr/> TEXT Foo</article> <hr/> </body>
in css don't hide hr directly, but do
.no_hr hr {
display:none;
}
now define a second style in your css
.block_hr hr{
display:block;
}
in your buttons onClick, change the one and only body class from no_hr to block_hr
onclick() {
if ( document.body.className == "no_hr" ) {
document.body.className = "block_hr";
} else {
document.body.className = "no_hr";
}
}
This is a very charming solution, because you don't have to iterate over elements yourself, but let your browsers optimized procedures do their job.

For people who want a solution that doesn't require JavaScript.
Create an invisible checkbox at the top of the document and make sure that people can click on it.
<input type="checkbox" id="ruler"/>
<label for="ruler">Click to show or hide the rules</label>
Then tell the stylesheet that the <hr>s should be hidden by default, but should be visible if the checkbox is checked.
#ruler, hr {display:none}
#ruler:checked ~ hr {display:block}
Done. See fiddle.

getElementsByTagName() returns a node list, and therefore you must iterate through all the results. Additionally, there is no innerHTML property of an <hr> tag, and the style must be set directly on the tag.
I like writing these types of iterations using Array.forEach() and call:
[].forEach.call(document.getElementsByTagName("hr"), function(item) {
item.style.display = "block";
});
Or, make it even easier on yourself and use jQuery:
$("hr").show();

Calculate the number of html element with js

I have multiple div elements with same id='mydiv'.I want to calculate these divs and Iam using the code
document.getElementById('mydiv').length
But it is not working

What you should do is use class instead of ID's. ID is for one element only, class is for multiple.
http://jsfiddle.net/d7AHV/

It won't work as getElementById will always return an element with the specified ID and null if the specified ID doesn't exist
From ECMA
getElementById(elementId) This method returns a Element. The elementId
parameter is of type DOMString.
What you can do is to assign each div with class
<div class="mydiv"></div>
<div class="mydiv"></div>
<div class="mydiv"></div>
<div class="mydiv"></div>
<div class="mydiv"></div>​​​​​​​​​
And iterate over:
var divs = document.getElementsByTagName('div');
var count = 0;
for(var i = 0; i < divs.length; i++) {
if(divs[i].className == 'mydiv') count++;
}
alert(count);
If your clients support document.getElementsByClassName(), it's even more concise:
alert(document.getElementsByClassName('mydiv').length)

You've been told about multiple elements with the same ID, but in rare cases it might be unavoidable (e.g. an XML document over which you have no control). The only adverse behaviour is that selecting by ID will usually only return the first one (but that's not guaranteed).
You can count elements with the same id by looping over all the elements in the document and counting the ones with a matching id, e.g.
function countSameIds(id) {
var allNodes = document.getElementsByTagName('*');
for (var i=allNodes.length, count=0; i; ) {
if (allNodes[--i].id == id) {
++count;
}
}
return count;
}

Find an element in DOM based on an attribute value

Can you please tell me if there is any DOM API which search for an element with given attribute name and attribute value:
Something like:
doc.findElementByAttribute("myAttribute", "aValue");

Modern browsers support native querySelectorAll so you can do:
document.querySelectorAll('[data-foo="value"]');
https://developer.mozilla.org/en-US/docs/Web/API/Document.querySelectorAll
Details about browser compatibility:
http://quirksmode.org/dom/core/#t14
http://caniuse.com/queryselector
You can use jQuery to support obsolete browsers (IE9 and older):
$('[data-foo="value"]');

Update: In the past few years the landscape has changed drastically. You can now reliably use querySelector and querySelectorAll, see Wojtek's answer for how to do this.
There's no need for a jQuery dependency now. If you're using jQuery, great...if you're not, you need not rely it on just for selecting elements by attributes anymore.
There's not a very short way to do this in vanilla javascript, but there are some solutions available.
You do something like this, looping through elements and checking the attribute
If a library like jQuery is an option, you can do it a bit easier, like this:
$("[myAttribute=value]")
If the value isn't a valid CSS identifier (it has spaces or punctuation in it, etc.), you need quotes around the value (they can be single or double):
$("[myAttribute='my value']")
You can also do start-with, ends-with, contains, etc...there are several options for the attribute selector.

We can use attribute selector in DOM by using document.querySelector() and document.querySelectorAll() methods.
for yours:
document.querySelector("[myAttribute='aValue']");
and by using querySelectorAll():
document.querySelectorAll("[myAttribute='aValue']");
In querySelector() and querySelectorAll() methods we can select objects as we select in "CSS".
More about "CSS" attribute selectors in https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors

FindByAttributeValue("Attribute-Name", "Attribute-Value");
p.s. if you know exact element-type, you add 3rd parameter (i.e.div, a, p ...etc...):
FindByAttributeValue("Attribute-Name", "Attribute-Value", "div");
but at first, define this function:
function FindByAttributeValue(attribute, value, element_type) {
element_type = element_type || "*";
var All = document.getElementsByTagName(element_type);
for (var i = 0; i < All.length; i++) {
if (All[i].getAttribute(attribute) == value) { return All[i]; }
}
}
p.s. updated per comments recommendations.

Use query selectors, examples:
document.querySelectorAll(' input[name], [id|=view], [class~=button] ')
input[name] Inputs elements with name property.
[id|=view] Elements with id that start with view-.
[class~=button] Elements with the button class.

Here's how you can select using querySelector:
document.querySelector("tagName[attributeName='attributeValue']")

Here is an example , How to search images in a document by src attribute :
document.querySelectorAll("img[src='https://pbs.twimg.com/profile_images/........jpg']");

you could use getAttribute:
var p = document.getElementById("p");
var alignP = p.getAttribute("align");
https://developer.mozilla.org/en-US/docs/Web/API/Element/getAttribute

Amendment for Daniel De León's Answer
It's possible to search with
^= - filters Elements where id (or any other attr) starts with view keyword
document.querySelectorAll("[id^='view']")

very simple, try this
<!DOCTYPE html>
<html>
<body>
<h1>The Document Object</h1>
<h2>The querySelector() Method</h2>
<h3>Add a background color to the first p element:</h3>
<p>This is a p element.</p>
<p data-vid="1">This is a p element.</p>
<p data-vid="2">This is a p element.</p>
<p data-vid="3">This is a p element.</p>
<script>
document.querySelector("p[data-vid='1']").style.backgroundColor = "red";
document.querySelector("p[data-vid='2']").style.backgroundColor = "pink";
document.querySelector("p[data-vid='3']").style.backgroundColor = "blue";
</script>
</body>
</html>

function optCount(tagId, tagName, attr, attrval) {
inputs = document.getElementById(tagId).getElementsByTagName(tagName);
if (inputs) {
var reqInputs = [];
inputsCount = inputs.length;
for (i = 0; i < inputsCount; i++) {
atts = inputs[i].attributes;
var attsCount = atts.length;
for (j = 0; j < attsCount; j++) {
if (atts[j].nodeName == attr && atts[j].nodeValue == attrval) {
reqInputs.push(atts[j].nodeName);
}
}
}
}
else {
alert("no such specified tags present");
}
return reqInputs.length;
}//optcount function closed
This is a function which is is used tu to select a particular tag with specific attribute value. The parameters to be passed are are the tag ID, then the tag name - inside that tag ID, and the attribute and fourth the attribute value.
This function will return the number of elements found with the specified attribute and its value.
You can modify it according to you.