Can JavaScript download files?

Blobs and object URLsexposed

File downloading is a core aspect of surfing the internet. Tons of files get downloaded from the internet every day ranging frombinary files(like applications, images, videos, and audios) to files in plain text.

Fetching files from theserver

Traditionally, the file to be downloaded is first requested from aserver through aclient  such as a users web browser. The server then returns a response containing thecontentof the file and some instructional headers specifying how the client should download the file.

In this diagram, the green line shows the flow of the request from the client to the server over HTTP. The orange line shows the flow of the response from the server back to the client.

Though the diagram indicates the communication flow, it does not explicitly show what the request from the client looks like or what the response from the server looks like.

Here is what the response from the server could possibly look like:

In this response, the server simply serves the raw content of the resource (represented with the asterisks  *) which will be received by the client.

The response also contains some headers that give the client some information about the nature of the content it receives  in this example response, theContent-TypeandContent-Lengthheaders provide that information.

When the client (web browser in this case) receives this HTTP response, it simply displays or renders the GIF image  which is not the desired behavior.The desired behavior is that the image should be downloaded not displayed.

Enforcing filedownload

To inform the client that the content of the resource is not meant to be displayed, the server must include an additional header in the response. TheContent-Dispositionheader is the right header for specifying this kind of information.

TheContent-Dispositionheader was originally intended for mail user-agents  since emails are multipart documents that may contain several file attachments. However, it can be interpreted by several HTTP clients including web browsers. This header provides information on thedisposition typeanddisposition parameters.

Thedisposition typeis usually one of the following:

1. inline  The body part is intended to be displayed automatically when the message content is displayed
2. attachment  The body part is separate from the main content of the message and should not be displayed automatically except when prompted by the user

Thedisposition parametersare additional parameters that specify information about the body part or file such as filename, creation date, modification date, read date, size, etc.

Here is what the HTTP response for the GIF image should look like to enforce file download:

Now the server enforces a download of the GIF image. Most HTTP clients will prompt the user to download the resource content when they receive a response from a server like the one above.

Click to download in thebrowser

Lets say you have the URL to a downloadable resource. When you try accessing that URL on your web browser, it prompts you to download the resource file  whatever the file is.

The scenario described above is not feasible in web applications. For web applications, the desired behavior will be  downloading a file in response to a user interaction. For example,click to save a photoordownload a report.

Achieving such a behavior in the browser is possible with HTMLanchor elements(<a></a>). Anchor elements are useful for adding hyperlinks to other resources and documents from an HTML document. The URL of the linked resource is specified in thehrefattribute of the anchor element.

Here is a conventional HTML anchor element linking to a PDF document:

The download attribute

In HTML 5, a newdownloadattribute was added to the anchor element. Thedownloadattribute is used to inform the browser to download the URL instead of navigating to it  hence a prompt shows up, requesting that the user saves the file.

Thedownloadattribute can be given a valid filename as its value. However, the user can still modify the filename in the save prompt that pops-up.

There are afew noteworthy factsabout the behavior of thedownloadattribute:

1. In compliance with thesame-origin policy, this attribute only works for same-origin URLs. Hence, it cannot be used to download resources served from a different origin
2. Besides HTTP(s) URLs, it also supportsblob:anddata:URLs  which makes it very useful for downloading content generated programmatically with JavaScript
3. For URLs with a HTTPContent-Dispositionheader that specifies a filename  the header filename has a higher priority than the value of thedownloadattribute

Here is the updated HTML anchor element for downloading the PDF document:

Programmatic content generation

With the advent of HTML5 and new Web APIs, it has become possible to do a lot of complex stuff in the browser using JavaScript without ever having to communicate with a server.

There are now Web APIs that can be used to programmatically:

• draw and manipulate images or video frames on a canvas  Canvas API
• read the contents and properties of files or even generate new data for files  File API
• generate object URLs for binary data  URL API

to mention only a few.

In this section, we will examine how we can programmatically generate content using Web APIs on the browser.

Lets consider two common examples.

Example 1  CSV generation from JSONarray

In this example, we will use theFetch APIto asynchronously fetch JSON data from a web service and transform the data to form a string ofcomma-separated-valuesthat can be written to a CSV file. Here is a breakdown of what we are about to do:

• fetch an array collection of JSON objects from an API
• extract selected fields from each item in the array
• reformat the extracted data as CSV

Here is what the CSV generation script could look like:

Here we are fetching a collection of photos from thePicsum Photos APIusing the globalfetch()function provided by theFetch API, filtering the collection and converting the collection array to a CSV string. The code snippet simply logs the resulting CSV string to the console.

First, we define asquareImagesfilter functionfor filtering images in the collection with equal width and height.

Next, we define acollectionToCSVhigher-order functionwhich takes an array of keys and returns a function that takes an array collection of objects and converts it to a CSV string extracting only the specified keys from each object.

Finally, we specify the fields we want to extract from each photo object in the collection in theexportFieldsarray.

Here is what the output could look like on the console:

Example 2  Image pixel manipulation using the CanvasAPI

In this example, we will use theCanvas APIto manipulate the pixels of an image, making it appear grayscale. Here is a breakdown of what we are about to do:

• set the canvas dimensions based on the image
• draw the image on a canvas
• extract and transform the image pixels on the canvas to grayscale
• redraw the grayscale pixels on the canvas

Lets say we have a markup that looks pretty much like this:

<div id=image-wrapper>
<canvas></canvas>
<img src=https://example.com/imgs/random.jpg&#8221; alt=Random Image>
</div>

Here is what the image manipulation script could look like:

Here is a comparison between an actual image and the corresponding grayscale canvas image.

Blobs and objectURLs

Before we proceed to learn how we can download content generated programmatically in the browser, lets take some time to look at a special kind of object interface calledBlob, which is already been implemented by most of the major web browsers. You can learn about Blobshere.

Blobs are objects that are used to represent raw immutable data.Blob objects store information about the type and size of data they contain, making them very useful for storing and working file contents on the browser. In fact, theFileobject is a special extension of theBlobinterface.

Obtaining blobs

Blob objects can be obtained from a couple of sources:

• Created from non-blob data using theBlobconstructor
• Sliced from an already existing blob object using theBlob.slice()method
• Generated from Fetch API responses or other Web API interfaces

Here are some code samples for the aforementioned blob object sources:

Reading blobcontent

It is one thing to obtain a blob object and another thing altogether to work with it. One thing you want to be able to do is to read the content of the blob. That sounds like a good opportunity to use aFileReaderobject. You can learn aboutFileReaderobjectshere.

AFileReaderobject provides some very helpful methods for asynchronously reading the content of blob objects or files in different ways. TheFileReaderinterface has pretty good browser support and supports reading blob data as follows(as at the time of this writing):

• as text  FileReader.readAsText()
• as binary string  FileReader.readAsBinaryString()
• as base64 data URL  FileReader.readAsDataURL()
• as array buffer  FileReader.readAsArrayBuffer()

Building on the Fetch API example we had before, we can use aFileReaderobject to read the blob as follows:

Object URLs

TheURLinterface allows for creating special kinds of URLs calledobject URLs, which are used for representing blob objects or files in a very concise format. Here is what a typical object URL looks like:

Creating and releasing objectURLs

TheURL.createObjectURL()static method makes it possible to create an object URL that represents a blob object or file. It takes a blob object as its argument and returns aDOMStringwhich is the URL representing the passed blob object. Here is what it looks like:

It is important to note that, this method will always return a new object URL each time it is called, even if it is called with the same blob object.

Whenever an object URL is created, it stays around for the lifetime of the document on which it was created. Usually, the browser will release all object URLs when the document is being unloaded. However, it is important that you release object URLs whenever they are no longer needed in order to improve performance and minimize memory usage.

TheURL.revokeObjectURL()static method can be used to release an object URL. It takes the object URL to be released as its argument. Here is what it looks like:

Using objectURLs

Object URLs can be used wherever a URL can be supplied programmatically. For example:

• they can be used to load files that can be displayed or embedded in the browser such as images, videos, audios, PDFs, etc  for example, by setting thesrcproperty of anImageelement
• they can be used as thehrefattribute of an<a></a>element, making it possible to download content that was extracted or generated programmatically

Downloading generated content

So far, we have looked at how we can download files that are served from a server and sent to the client over HTTP  which is pretty much thetraditional flow. We have also seen how we can programmatically extract or generate content in the browser using Web APIs.

In this section, we will examine how we can download programmatically generate content in the browser, leveraging all we have learned from the beginning of the article and what we already know about blobs and object URLs.

Creating the downloadlink

First, lets say we have ablob objectby some means. We want to create a helper function that allows us to create a download link (<a></a>element) that can be clicked in order to download the content of the blob, just like a regular file download.

The logic of our helper function can be broken down as follows:

• Create an object URL for the blob object
• Create ananchorelement (<a></a>)
• Set thehrefattribute of the anchor element to the created object URL
• Set thedownloadattribute to the filename of the file to be downloaded. This forces the anchor element to trigger a file download when it is clicked
• If the link is for a one-off download, release the object URL after the anchor element has been clicked

Here is what an implementation of this helper function will look like:

That was a pretty straightforward implementation of the download link helper function. Notice that the helper triggers aone-off automaticdownload of the blob content whenever it is called.

Also notice that the helper function takes a filename as its second argument, which is very useful for setting the default filename for the downloaded file.

The helper function returns a reference to the created anchor element (<a></a>), which is very useful if you want to attach it to the DOM or use it in some other way.

Here is a simple example:

Revisiting theexamples

Now that we have our download helper function in place, we can revisit our previous examples and modify them to trigger a download for the generated content. Here we go.

1. CSV generation from JSONarray

We will update the final promise.thenhandler to create a download link for the generated CSV string and automatically click it to trigger a file download using thedownloadBlobhelper function we created in the previous section.

Here is what the modification should look like:

Here we have updated the final promise.then handler as follows:

• create a new blob object for the CSV string, also setting the correct type using:

• call thedownloadBlobhelper function to trigger an automatic download for the CSV file, specifying the default filename asphotos.csv
• move the promise rejection handler to a separate.catch()block:

Here is a working and more advanced example of this application on Codepen:

See the Pen
JSON Collection to CSV by Glad Chinda (@gladchinda)
on CodePen.

2. Image pixel manipulation

We will add some code to the end of theloadevent listener of theimgobject, to allow us:

• create a blob object for the grayscale image in thecanvasusing theCanvas.toBlob()method
• and then create a download link for the blob object using ourdownloadBlobhelper function from before
• and finally, append the download link to the DOM

Here is what the update should look like:

Here is a working example of this application onCodepen:

See the Pen
Image Pixel Manipulation Grayscale by Glad Chinda (@gladchinda)
on CodePen.

Conclusion

Weve finally come to the end of this tutorial. While there could be a lot to pick from this tutorial, it is glaring that Web APIs have a lot to offer as regards building powerful apps for the browser. Dont hesitate to be experimental and adventurous.

Thanks for making out time to read this article. If you found this article insightful, feel free to give some rounds of applause if you dont mind  as that will help other people find it easily on Medium.