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audio file and json file reading methods difference in javascript

I find difficult to understand one methodological thing about javascript coding.
When in a script I want to read an audio file, I do it like this:
var audio = new Audio('theme.mp3');
audio.play();
It is simple, clear and works dynamically: I have an access to a file system from the site (is it true?).
But if I'd like to read a json file, which is substantially a text file, I have to write an AJAX request:
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
var myObj = JSON.parse(this.responseText);
//code
}
};
xmlhttp.open("GET", "json_demo.txt", true);
xmlhttp.send();
And also using jQuery we see the same but shorten thing.
Could you explain me, why I can read all the graphics, audio files etc, but to upload JSON file I have to do that request? How does this kind of communication between client and server work? Where is a difference? Why I can't just read the text file, like I did it above with theme.mp3?
Thank you so much.
UPDATE:
The most important thing: when I read the audio file as above, the script is working nicely even if I open the html file from the computer. At the same time the loading of JSON file doesn't work until the page is opened from the localhost. (I'm sorry if I use incorrect terms).
"Help me StackOverflow, you are the my last hope."

How to read file without file input?

I'm developing an application in Javascript, which requires to read a text file and put it into a variable as input. However, I google many related questions and found that FileReader can't read file without file input because of security reason. Is there any library or dirty ways to read the content of a text file with only local path? Thanks a lot.

I think a way to do it would be to load your file with Ajax. Then you can do whatever you want with its content.
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
// Do whatever you want here with this.responseText
}
};
xhttp.open("GET", "youtext.txt", true);
xhttp.send();

It's easier to put your file.txt or file.json to your localhost, or put it on your server.
or this link maybe can help you out
https://github.com/nwjs/nw.js/issues/3227
i understand this question that he want to read file line by line from his local path.

javascript - reading in an external local .txt file to load data into an array

I currently have javascript code (please see below) that searches an array for a month/day combination, and if it is found, assigns the name of a .jpg file (which is used for the background image of a page). Instead of hard-coding all of the data in the array, I'd like to be able to create an external .txt file with month/day codes and associated image file names, that could be read and loaded into the array. Thanks for your help!
var ourdates = ['0000','0118','0215','0530','0614','0704','0911','1111','1207']
if (ourdates.indexOf(monthday) != -1)
{
ourimage = "flag";
}

If you mean loading it from your server, that's a classic use-case for ajax, frequently combined with JSON:
var ourdates = null;
var xhr = new XMLHttpRequest();
xhr.open("GET", "/path/to/your/data");
xhr.onreadystatechange = function() {
if (xhr.readyState == 4 && xhr.status == 200) {
ourdates = JSON.parse(xhr.responseText);
// call something that uses `ourdates`
}
};
xhr.send(null)
If you mean from the user's computer, my answer here shows how to do that with the File API. Doing that requires an <input type="file"> input (or drag-and-drop event) that the user uses to grant access to the file to your script. You can't read a file from their machine without them specifically giving you access to the file.

Load a 'server-side' file with XMLHttpRequest in local html file

I want to load a json-stringified file in my javascript. The javascript reside in a html-file which I load from my local file system.
I have tried with the following code:
var xhr = new XMLHttpRequest();
xhr.open('GET', fileName, true);
xhr.responseType = 'blob';
xhr.onload = function(e) {
if (this.status == 200) {
// get binary data as a response
var blob = this.response;
alert("Yo");
}
};
But the onload event fires only once, with the status=0, then no more happens.
I have tried to use both a full path to the file as well as a local file path like "/files/the_file.txt".
It looks like the problem is related with me trying to run the html file locally. I don't want to set-up a local server as I have seen proposed in similar posts here at so.
Anyone out there with a solution to this problem?
EDIT:
This is not what I want, but this might serve to give an example of how I almost want it. This example let the user select a file, and my script can now access the content of the selected file.
HTML:
<input type="file" id="FancyInputField" onchange="doIt();">
Javascript:
function doIt(){
var selectedFile = document.getElementById('FancyInputField').files[0];
reader = new FileReader();
reader.onload = function (e) {
var output = reader.result;
var daObject = JSON.parse(output);
}
reader.readAsText(selectedFile);
}
This also works with a local html file. (No local server)
My question stands; How do I read the file(s) with no user interaction? The files reside in a sub-folder to where the html file are located. I can with no problem load and show an image from the same sub-folder, with an <img> tag. ..So why is it so difficult to load a text file?

How do I read the file(s) with no user interaction?
You can't. Those files belong to the user, not your website. You can't choose to read them.
I can with no problem load and show an image from the same sub-folder, with an <img> tag
There is a lot of difference between displaying an image to the user, and making the content of a file available to JavaScript code written by the page author.
So why is it so difficult to load a text file?
Send someone an HTML document in an email
Enjoy the JavaScript in it scooping up files from the hard disk and sending them to Joe Evil Hacker's server
It's just basic security.

Use URL.createObjectURL(file), instead of ajax.

How to load a PDF into a blob so it can be uploaded?

I'm working on a testing framework that needs to pass files to the drop listener of a PLUpload instance. I need to create blob objects to pass inside a Data Transfer Object of the sort generated on a Drag / Drop event. I have it working fine for text files and image files. I would like to add support for PDF's, but it seems that I can't get the encoding right after retrieving the response. The response is coming back as text because I'm using Sahi to retrieve it in order to avoid Cross-Domain issues.
In short: the string I'm receiving is UTF-8 encoded and therefore the content looks like you opened a PDF with a text editor. I am wondering how to convert this back into the necessary format to create a blob, so that after the document gets uploaded everything looks okay.
What steps do I need to go through to convert the UTF-8 string into the proper blob object? (Yes, I am aware I could submit an XHR request and change the responseType property and (maybe) get closer, however due to complications with the way Sahi operates I'm not going to explain here why I would prefer not to go this route).
Also, I'm not familiar enough but I have a hunch maybe I lose data by retrieving it as a string? If that's the case I'll find another approach.
The existing code and the most recent approach I have tried is here:
var data = '%PDF-1.7%����115 0 obj<</Linearized 1/L ...'
var arr = [];
var utf8 = unescape(encodeURIComponent(data));
for (var i = 0; i < utf8.length; i++) {
arr.push(utf8.charCodeAt(i));
}
var file = new Blob(arr, {type: 'application/pdf'});

It looks like you were close. I just did this for a site which needed to read a PDF from another website and drop it into a fileuploader plugin. Here is what worked for me:
var url = "http://some-websites.com/Pdf/";
//You may not need this part if you have the PDF data locally already
var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function () {
if (this.readyState == 4 && this.status == 200) {
//console.log(this.response, typeof this.response);
//now convert your Blob from the response into a File and give it a name
var fileOfBlob = new File([this.response], 'your_file.pdf');
// Now do something with the File
// for filuploader (blueimp), just use the add method
$('#fileupload').fileupload('add', {
files: [ fileOfBlob ],
fileInput: $(this)
});
}
}
xhr.open('GET', url);
xhr.responseType = 'blob';
xhr.send();
I found help on the XHR as blob here. Then this SO answer helped me with naming the File. You might be able to use the Blob by itself, but you won't be able to give it a name unless its passed into a File.