I need to serialize a File object from a file input, so that the object can be saved, parsed back to a file object, and then read using the FileReader object.
Does anyone know if this is possible in Google Chrome?
I think the problem lies in the protection of the file.path property. Webkit browsers hide this property, so I am guessing when you serialize it, the path is removed.
Then of course, the FileReader is unable to read it without path information.
Here is an example:
var files = uploadControl.files[0];
var dataFile = JSON.stringify(files);
var newFile = JSON.parse(dataFile);
var reader = new FileReader();
reader.onload = (function(event) {
var fileContents = event.target.result;
});
reader.readAsText(newFile);
Nothing happens. The reader is not loaded. If I pass the JSON object, it doesn't work either.
As a matter of principle, what you are asking for will not be possible. If it were possible to have some text which represented the file object, then you could construct that text from scratch, unserialize it, and thus get access to files the user did not grant permission to.
(The exception to this is if the representative text is some sort of robustly-secret bitstring (cryptographically signed, or a sparse key in a lookup table) that only the browser could have given out in the first place — but I expect if that feature existed, it would be documented and you would have found it already.)
Related
I am new to this stuff.
I'm trying to build a website about finance.
I have written a scrypt in python to get shares data from an API, created my own market index and exported the datas of my index in a CSV file. It's important because, I need to create an historic and observe its evolution (the script will run automatically in a VM to add data as it goes). No problem at this stage.
Now, my purpose is to read and treat my CSV file in a JS script to stock the datas into an array (graphJS use array to display a graph and that's what I want to use).
I only found solutions that use an <input> HTML (user import a CSV on a website, I do not need that) or JSQuery (I have never been able to use it)
I'm looking for a native JavaScript solution. For the beginning, I am just looking to display datas from my CSV in console.log(). And with a "for" loop, use ".push" to iterate the datas into an array. But I will do this after I successfully displayed my file in the console.
My code does not work, can someone help me ?
const csvLocalFile = "file.csv";
const csv = csvLocalFile.files[0];
const reader = new FileReader();
reader.readAsText(csv);
TypeError: Cannot read properties of undefined (reading '0')
The Issue Is That You Are Trying To Access The File Contents From A Variable Of Type String.
What Your Code Does Is Try To Reference An Element Inside This String Like A Map Which doesn't Exist. Giving The Error.
Also In The Code It Only Creates A New Variable whose contents are file.csv and doesn't actually store the file.
To Actually Get The Contents Of Your CSV:
var filecontent
// https://stackoverflow.com/a/29176118
var openFile = function(event) {
var input = event.target;
var reader = new FileReader();
reader.onload = function(){
var text = reader.result;
filecontent = text.replace("","")
};
reader.readAsText(input.files[0]);
};
And In Your HTML
<input type="file"onchange="openFile(event)">
Your CSV Contents Will Be Stored Inside the Variable filecontent
EDIT: Non File Upload Types
let obj = await (await fetch('file.csv')).text();
This Contains Your CSV.
I want to read and write to a file in a specific way.
An example file could be:
name1:100
name2:400
name3:7865786
...etc etc
What would be the best way to read this data in and store in, and eventually write it out?
I don't know which type of data structure to use? I'm still fairly new to javascript.
I want to be able to determine if any key,values are matching.
For example, if I were to add to the file, I could see that name1 is already in the file, and I just edit the value instead of adding a duplicate.
You can use localStorage as a temporary storage between reads and writes.
Though, you cannot actually read and write to a user's filesystem at will using client side JavaScript. You can however request the user to select a file to read the same way you can request the user to save the data you push, as a file.
localStorage allow you to store the data as key-value pairs and it's easy to check if an item exists or not. Optionally simply use a literal object which basically can do the same but only exists in memory. localStorage can be saved between sessions and navigation between pages.
// set some data
localStorage.setItem("key", "value");
// get some data
var data = localStorage.getItem("key");
// check if key exists, set if not (though, you can simply override the key as well)
if (!localStorage.getItem("key")) localStorage.setItem("key", "value");
The method getItem will always return null if the key doesn't exist.
But note that localStorage can only store strings. For binary data and/or large sizes, look into Indexed DB instead.
To read a file you have to request the user to select one (or several):
HTML:
<label>Select a file: <input type=file id=selFile></label>
JavaScript
document.getElementById("selFile").onchange = function() {
var fileReader = new FileReader();
fileReader.onload = function() {
var txt = this.result;
// now we have the selected file as text.
};
fileReader.readAsText(this.files[0]);
};
To save a file you can use File objects this way:
var file = new File([txt], "myFilename.txt", {type: "application/octet-stream"});
var blobUrl = (URL || webkitURL).createObjectURL(file);
window.location = blobUrl;
The reason for using octet-stream is to "force" the browser to show a save as dialog instead of it trying to show the file in the tab, which would happen if we used text/plain as type.
So, how do we get the data between these stages. Assuming you're using key/value approach and text only you can use JSON objects.
var file = JSON.stringify(localStorage);
Then send to user as File blob shown above.
To read you will have to either manually parse the file format if the data exists in a particular format, or if the data is the same as you save out you can read in the file as shown above, then convert it from string to an object:
var data = JSON.parse(txt); // continue in the function block above
Object.assign(localStorage, data); // merge data from object with localStorage
Note that you may have to delete items from the storage first. There is also the chance other data have been stored there so these are cases that needs to be considered, but this is the basis of one approach.
Example
// due to security reasons, localStorage can't be used in stacksnippet,
// so we'll use an object instead
var test = {"myKey": "Hello there!"}; // localStorage.setItem("myKey", "Hello there!");
document.getElementById("selFile").onchange = function() {
var fileReader = new FileReader();
fileReader.onload = function() {
var o = JSON.parse(this.result);
//Object.assign(localStorage, o); // use this with localStorage
alert("done, myKey=" + o["myKey"]); // o[] -> localStorage.getItem("myKey")
};
fileReader.readAsText(this.files[0]);
};
document.querySelector("button").onclick = function() {
var json = JSON.stringify(test); // test -> localStorage
var file = new File([json], "myFilename.txt", {type: "application/octet-stream"});
var blobUrl = (URL || webkitURL).createObjectURL(file);
window.location = blobUrl;
}
Save first: <button>Save file</button> (<code>"myKey" = "Hello there!"</code>)<br><br>
Then read the saved file back in:<br>
<label>Select a file: <input type=file id=selFile></label>
Are you using Nodejs? Or browser javascript?
In either case the structure you should use is js' standard object. Then you can turn it into JSON like this:
var dataJSON = JSON.stringify(yourDataObj)
With Nodejs, you'll want to require the fs module and use one of the writeFile or appendFile functions -- here's sample code:
const fs = require('fs');
fs.writeFileSync('my/file/path', dataJSON);
With browser js, this stackoverflow may help you: Javascript: Create and save file
I know you want to write to a file, but but consider a database instead so that you don't have to reinvent the wheel. INSERT ... ON DUPLICATE KEY UPDATE seems like the logical choice for what you're looking to do.
For security reasons it's not possible to use JavaScript to write to a regular text or similar file on a client's system.
However Asynchronous JavaScript and XML (AJAX) can be used to send an XMLHttpRequest to a file on the server, written in a server-side language like PHP or ASP.
The server-side file can then write to other files, or a database on the server.
Cookies are useful if you just need to save relatively small amounts of data locally on a client's system.
For more information have a look at
Read/write to file using jQuery
I am reading a local CSV file using a web UI, and the HTML5 FileReader interface to handle the local file stream. This works great.
However, sometimes I want the file being read to be updated continuously, after the initial load. I am having problems, and I think it might have something to do with the FileReader API. Specifically, after the initial file load, I maintain a reference to the file. Then, when I detect that the size of the file has increased, I slice off the new part of the file, and get a new Blob object. However, there appears to be no data in these new Blobs.
I am using PapaParse to handle the CSV parsing, though I don't think that is the source of the problem (though it may be).
The source code is too voluminous to post here, but here is some pseudocode:
var reader = new FileReader();
reader.onload = loadChunk;
var file = null;
function readLocalFile(event) {
file = event.target.files[0];
// code that divides file up into chunks.
// for each chunk:
readChunk(chunk);
}
function readChunk(chunk) {
reader.readAsText(chunk);
}
function loadChunk(event) {
return event.target.result;
}
// this is run when file size has increased
function readUpdatedFile(oldLength, newLength) {
var newData = file.slice(oldLength, newLength);
readChunk(newData);
}
The output of loadChunk when the file is first loading is a string, but after the file has been updated it is a blank string. I am not sure if the problem is with my slice method, or if there is something going on with FileReader that I am not aware of.
The spec for File objects shouldn't allow this: http://www.w3.org/TR/FileAPI/#file -- it's supposed to be like a snapshot.
The fact that you can detect that the size has changed is probably a shortcoming of an implementation.
i'm currently trying to upload a file to my server. But i'm not really sure how to do this with readAsArrayBuffer. This works if I use readAsBinaryString.
If i try to console.log it only returns 'arrayBuffer: {}'.
After I've tried to upload it, i can see inside post that only a empty object was sent. If I use readAsBinaryString, I see a bunch of binary code.
var file = document.getElementById('my_file').files[0],
reader = new FileReader();
reader.onloadend = function(e){
console.log(e.target.result);
$scope.image = e.target.result;
}
reader.readAsArrayBuffer(file);
How can I see my file, so I know it's working when using readAsArrayBuffer?
If more code is needed let me know! Thanks.
According to ArrayBuffer documentation
You can not directly manipulate the contents of an ArrayBuffer; instead, you create one of the typed array objects or a DataView object which represents the buffer in a specific format, and use that to read and write the contents of the buffer.
So as I commented before probably console.log doesn't know how to represent the buffer and therefore it simply outputs arrayBuffer: {}.
If you want to show something in the console you need to use a typed array or a DataView. For example using an Int8Array:
reader.onloadend = function (e) {
console.log(e);
console.log(new Int8Array(e.target.result));
};
See demo
If you want to upload am image then you have to convert it into base64 format. You can do it either by using canvas element or by using Filereader.If you are using Filereader then you have to use readAsDataURL()
You can refer MDN for this
https://developer.mozilla.org/en-US/docs/Web/API/FileReader.readAsDataURL
also you can use canvas element
Convert an image to canvas that is already loaded
I have an HTML5/javscript app which uses
<input type="file" accept="image/*;capture=camera" onchange="gotPhoto(this)">
to capture a camera image. Because my app wants to be runnable offline, how do I save the File (https://developer.mozilla.org/en-US/docs/Web/API/File) object in local storage, such that it can be retrieved later for an ajax upload?
I'm grabbing the file object from the using ...
function gotPhoto(element) {
var file = element.files[0];
//I want to save 'file' to local storage here :-(
}
I can Stringify the object and save it, but when I restore it, it is no longer recognised as a File object, and thus can't be used to grab the file content.
I have a feeling it can't be done, but am open to suggestions.
FWIW My workaround is to read the file contents at store time and save the full contents to local storage. This works, but quickly consumes local storage since each file is a 1MB plus photograph.
You cannot serialize file API object.
Not that it helps with the specific problem, but ...
Although I haven't used this, if you look at the article it seems that there are ways (although not supported yet by most browsers) to store the offline image data to some files so as to restore them afterward when the user is online (and not to use localStorage)
Convert it to base64 and then save it.
function gotPhoto(element) {
var file = element.files[0];
var reader = new FileReader()
reader.onload = function(base64) {
localStorage["file"] = base64;
}
reader.readAsDataURL(file);
}
// Saved to localstorage
function getPhoto() {
var base64 = localStorage["file"];
var base64Parts = base64.split(",");
var fileFormat = base64Parts[0].split(";")[1];
var fileContent = base64Parts[1];
var file = new File([fileContent], "file name here", {type: fileFormat});
return file;
}
// Retreived file object
Here is a workaround that I got working with the code below. I'm aware with your edit you talked about localStorage but I wanted to share how I actually implemented that workaround. I like to put the functions on body so that even if the class is added afterwards via AJAX the "change" command will still trigger the event.
See my example here: http://jsfiddle.net/x11joex11/9g8NN/
If you run the JSFiddle example twice you will see it remembers the image.
My approach does use jQuery. This approach also demonstrates the image is actually there to prove it worked.
HTML:
<input class="classhere" type="file" name="logo" id="logo" />
<div class="imagearea"></div>
JS:
$(document).ready(function(){
//You might want to do if check to see if localstorage set for theImage here
var img = new Image();
img.src = localStorage.theImage;
$('.imagearea').html(img);
$("body").on("change",".classhere",function(){
//Equivalent of getElementById
var fileInput = $(this)[0];//returns a HTML DOM object by putting the [0] since it's really an associative array.
var file = fileInput.files[0]; //there is only '1' file since they are not multiple type.
var reader = new FileReader();
reader.onload = function(e) {
// Create a new image.
var img = new Image();
img.src = reader.result;
localStorage.theImage = reader.result; //stores the image to localStorage
$(".imagearea").html(img);
}
reader.readAsDataURL(file);//attempts to read the file in question.
});
});
This approach uses the HTML5 File System API's to read the image and put it into a new javascript img object. The key here is readAsDataURL. If you use chrome inspector you will notice the images are stored in base64 encoding.
The reader is Asynchronous, this is why it uses the callback function onload. So make sure any important code that requires the image is inside the onLoad or else you may get unexpected results.
You could use this lib:
https://github.com/carlo/jquery-base64
then do something similar to this:
//Set file
var baseFile = $.base64.encode(fileObject);
window.localStorage.setItem("file",basefile);
//get file
var outFile = window.localStorage.getItem("file");
an other solution would be using json (I prefer this method)
using: http://code.google.com/p/jquery-json/
//Set file
window.localStorage.setItem("file",$.toJSON(fileobject));
//get file
var outFile = $.evalJSON(window.localStorage.getItem("file"));
I don't think that there is a direct way to Stringify and then deserialize the string object into the object of your interest. But as a work around you can store the image paths in your local storage and load the images by retrieving the URL for the images. Advantages would be, you will never run out of storage space and you can store 1000 times more files there.. Saving an image or any other file as a string in local storage is never a wise decision..
create an object on the global scope
exp: var attmap = new Object();
after you are done with file selection, put your files in attmap variable as below,
attmap[file.name] = attachmentBody;
JSON.stringify(attmap)
Then you can send it to controller via input hidden or etc. and use it after deserializing.
(Map<String, String>)JSON.deserialize(attachments, Map<String,String>.class);
You can create your files with those values in a for loop or etc.
EncodingUtil.base64Decode(CurrentMapValue);
FYI:This solution will also cover multiple file selection
You could do something like this:
// fileObj = new File(); from file input
const buffer = Buffer.from(await new Response(fileObj).arrayBuffer());
const dataUrl = `data:${fileObj.type};base64,${buffer.toString("base64")}`;
localStorage.setItem('dataUrl', dataUrl);
then you can do:
document.getElementById('image').src = localStorage.getItem('dataUrl');