How to get source code of html page

Using JavaScript

document.documentElement.outerHTML

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or

document.documentElement.innerHTML

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you can use the html method with jquery, i.e for get the whole page html like this:

$( document ).ready(function() {
    console.log($("html").html());
})

I'm not sure what you want to do exactly but if you want to get raw source code from jQuery you can use the following:

var html = $('element').html();

And with pure javascript from an id

var html = document.getElementById('id').innerHTML;

or from classname

var html = document.getElementsByClassName('class').innerHTML;

And to get the content of your example (which is an editor called tinymce) you can use the command tinymce.activeEditor.getContent(); or tinyMCE.get('myTextarea').getContent()

EDIT: If you want to listen for changes with jQuery and display to html dynamically you'd want to do something like this:

$('yourTextArea').keyup(function() {
    var html = $(this).val();
    $('yourElementToDisplayTheHTML').html(html);
});

I've built a simple API, just use this:

<script src="https://cdn.jsdelivr.net/gh/Parking-Master/viewsource@latest/vs.js"></script>

In your JavaScript:

getSource('https://example.com/');

in http response header, you can find "content-type" header. that header indicates how contents of page should be rendered. so, if you want to render page as plain text("source code", as you mentioned), just change that header like this -> "content-type : text/plain; charset=UTF-8"

if you can use ajax in jquery or javascript

use like this

$("button").click(function(){
$.ajax({url: "page_you_want.html", success: function(result){
    alert(result);
}});

});