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How can I add data params to an HTML string and get a string back?

I got a string like this:
var select_string = '<select><option>1</option><option>2</option></select>';
I need to add some data params to select in this string and get this string back in order to get the following:
select_string = '<select data-param1="param1" data-param2="param2"><option>1</option><option>2</option></select>';
I tried to use jQuery functions like .html() or .text() but it did not work. Like this:
select_string = $(select_string).data('param1', 'param1').html() //or .text()
Any ideas how to make it work would be helpful. Thank you.

You can use attr to add that attributes to the element
var select_string = '<select><option>1</option><option>2</option></select>';
var select = $(select_string).attr({
'data-param1': 'param1',
'data-param2': 'param2'
});
console.log(select.prop('outerHTML'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
Since your attribute name starts with data-, if you want to get the value, you can use:
select.data('param1'); // param1
select.data('param2'); // param2

EDIT: Titulum is right, jquery is not needed here.
But here is the working example usign jquery
var selectString = '<select><option>1</option><option>2</option></select>';
var $select = $(selectString);
$select.attr("prop_key","prop_value");
var selectChanged = $select.prop('outerHTML');
console.log(selectChanged)
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

You don't need jQuery for this:
const myElement = document.createElement('div');
myElement.innerHTML = "<select><option>1</option><option>2</option></select>";
const selectElement = myElement.getElementsByTagName("select")[0];
selectElement.setAttribute("data-param1", "param1");
selectElement.setAttribute("data-param2", "param2");

You could use indexOf and substr() to split it into 2 parts, insert your new text, and put it back together again.
var first_half = select_string.substr(0, select_string.indexOf('>'));
var second_half = select_string.substr(select_string.indexOf('>'));
select_string = first_half + ' data-param1=\"param1\" data-param2=\"param2\" ' + second_half;

How to extract all hyperlink titles from big html string using javascript?

I got an HTML string as :var code; I want to extract all hyper link title values in this big string and place them in textarea. I tried the following but it never works. could any one tell me what i am doing wrong?
sample hyperlinks to look for(i want to extract mango,cherry,...) :
mango
cherry
my code string has blocks of data like below:
<div class="details">
<div class="title">
mango
<span class="type">3</span>
</div>
</div>
full code:
$.getJSON('http://anyorigin.com/get?url=http://asite.com/getit.php/&callback=?', function(data){
//$('#output').html(data.contents);
var siteContents = data.contents;
//writes to textarea
document.myform.outputtext.value = siteContents ;
var start = siteContents.indexOf('<ul class="list">');
var end = siteContents.indexOf('<ul class="pag">', start);
var code = siteContents.substring(start, end);
document.myform2.outputtext2.value = code ;
var pattern = /<a href="([^"]+?)">([^<]+?)<\/a>/gi;
code = code.match(pattern);
for (i = 0; i < code.length; i++) {
document.write($2<br />'));
}
});
</script>

It looks like you're trying to parse HTML with regex. This post has some more info on that topic.
Since this question is tagged as jQuery, you could try something like the following...
Make a jQuery object out of the returned HTML:
$markup = $(data.contents);
Find the anchors:
$anchors = $markup.find('a');
Get the text (or whatever attribute you want from it):
arrText = [];
$anchors.each(function() {
arrText.push($(this).text());
});
Put result into textarea:
$textarea.val(arrText.join(','));

To achive this jquery is the simplest solution, you can try below code
$('a').each(function(){
var copiedTitle = $(this).html();
var previous = $('#test').html();
var newText = previous +"\n"+ copiedTitle;
$('#test').html(newText);
});
JS Fiddle

Using Variables in Replace() Method - Javascript

I am trying to replace the contents of the alt="" attribute in the tag.
The replacment text comes from textarea input that is assigned to var alttext
The var oldtext contains tags with placeholders for replacing, like:
<img alt="placeholder" scr="pic.jpg" />
The placeholder needs to be replaced the contents of var alttext.
So far I have tried:
function replacer() {
var alttext = document.myform.alttext.value;
var oldtext = document.myform.oldtext.value;
var replacedtext = oldtext.replace("placeholder", 'alttext' )
document.myform.outputtext.value = replacedtext;
}
But it does not work.
How can the alttext variable contents be used to replace the placeholder?
Thank you very much to everyone!

function replacer() {
var alttext = document.myform.alttext.value;
var oldtext = document.myform.oldtext.value;
var replacedtext = oldtext.replace("placeholder", alttext);
document.myform.outputtext.value = replacedtext;
}
you were trying to replace with quotes around your variable (alttext) making it a string literal

regular expression replace with prototype?

I got some html formatted in the following way:
[Title|<a class="external" href="http://test.com">http://test.com</a>]
From these texts I'd like to create links using "Title" as the text and "http://test.com" as link. How can I best do this in prototype?

Pure RegExp:
var ProperLink=WierdString.replace(/\[([^|]+)\|(<[^>]+>)[^<]+[^\]]+\]/,'$2$1</a>')
in the context you provided:
function convert(id){
$(id).innerHTML=$(id).innerHTML.replace(/\[([^|]+)\|(<[^>]+>)[^<]+[^\]]+\]/g,'$2$1</a>');
}
convert('testdiv');

Here is a regex that will retain the original attributes of the anchor tag while doing the replacement:
var link = "[Title|<a class=\"external\" href=\"http://test.com\">http://test.com</a>]";
var pattern = /\[([^|]+)\|([^>]+.?)[^<]*(<\/a>)\]/;
link.replace(pattern, "$2$1$3"));
The output is:
<a class="external" href="http://test.com">Title</a>

Without prototype: http://jsfiddle.net/JFC72/ , you can use prototype to make it simpler.
var myStr = "[THIS IS TITLE|http://test.com]";
document.getElementById('testdiv').innerHTML = getLink(myStr);
function getLink(myStr)
{
var splitted = myStr.split("|http");
var title = splitted[0].substring(1);
var href = splitted[1].substring(0,splitted[1].length-1);
return "<a href='http" + href + "'>" + title + "</a>";
}

var dummyDiv = document.createElement('div');
dummyDiv.innerHTML = '[Title|<a class="external ...';
var parts = dummyDiv.innerText.slice(1, -1).split('|');
// parts[0] is the text, parts[1] is the URL

Get source of image

I have a next string like:
<img src="../uplolad/commission/ranks/avatar.jpg' . $row[$c_name] .'" width="50" height="50"/>
How can i get a image file name in javascript? I know only PHP regexes. Extention of a file can be different.
The result must be: avatar.jpg

Regex is not ideal for this. JavaScript can traverse the HTML as distinct objects more readily than as a long string. If you can identify the picture by anything, say by adding an ID to it, or an ID to a parent with that as the only image, you'll be able to access the image from script:
var myImage = document.getElementById('imgAvatar'); // or whatever means of access
var src = myImage.src; // will contain the full path
if(src.indexOf('/') >= 0) {
src = src.substring(src.lastIndexOf('/')+1);
}
alert(src);
And if you want to edit, you can do that just as well
myImage.src = src.replace('.jpg', '.gif');

Fetch it following coding which can help what you want to get.
<script type="text/javascript">
function getImageName(imagePath) {
var objImage = new RegExp(/([^\/\\]+)$/);
var getImgName = objImage.exec(imagePath);
if (getImgName == null) {
return null;
}
else {
return getImgName[0];
}
}
</script>
<script>
var mystring = getImageName("http://www.mypapge.mm/myimage.png")
alert(mystring)
</script>

Here's a shorter variation of David Hedlund's answer that does use regex:
var myImage = document.getElementById('imgAvatar'); // or whatever means of access
alert(myImage.src.replace( /^.+\// , '' ));