FileReader API

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In my previous postI introduced using files in JavaScript, focusing specifically on how to get access to File objects. These objects contain file metadata obtained only when the user opts to either upload a file or drags and drops a file onto the web page. Once you have files, however, the next step is to read data from them. The FileReader type has a single job: The read is done asynchronously so as not to block the browser.

There are several formats that a FileReader can best binary file viewer javascript filereader to represent the file data, and the format must be requested when asking the file to be read. Reading is done through calling one of these methods:. As such, you must listen for the load event before starting to read. The result of the read is always represented by event. This example simply reads the contents of a file and outputs it in plain text to the console. The FileReader instance is available inside of the event handler via event.

The result property contains the file contents on success and error contains error information about the failed operation. You can use the same basic setup for reading to a data URI. Data URIs sometimes called data URLs are an interesting option if you want to, for example, display an image that was just read from disk.

You could do so with the following code:. This code simply inserts an image that was read from disk into a page. Since the data URI contains all of the image data, it can be passed directly into best binary file viewer javascript filereader src attribute of best binary file viewer javascript filereader image and displayed on the page.

This code loads the image data into a new Image object and then uses that to draw the image onto a canvas specifying both the width and height as Data URIs are generally used for this purpose, but can be used on any type of the file. The most common use case for reading a file into a data URI is to display the file contents on a web page immediately. An ArrayBuffer represents a finite number of bytes that may be used to store numbers of any size.

The way data is read from best binary file viewer javascript filereader ArrayBuffer is best binary file viewer javascript filereader using a specific view, such as Int8Arraywhich treats the underlying bytes as a collection of 8-bit signed integers or Float32Arraywhich treats the underlying bytes as a collection of bit floating point numbers.

These are called typed arrays 2which force you to work with a specific numeric type rather than containing any type of data as with traditional arrays. You use an ArrayBuffer primarily when dealing with binary files, to have more fine-grained control over the data. Reading data from a file using a FileReader is pretty simple. Any viewpoints best binary file viewer javascript filereader opinions expressed in this article are those of Nicholas C.

Zakas and do not, in any way, reflect those of my employer, my colleagues, Wrox PublishingO'Reilly Publishingor anyone else. I speak only for myself, not for them. This site is reader-supported. You can support my work in a number of ways: Your support is greatly appreciated. All code examples on all pages, unless otherwise indicated, are BSD licensed. Skip to content Writing Speaking About Contact. Working with files in JavaScript, Part 2: Reading is done through calling one of these methods: More complicated than you think Mimicking npm script in Node.

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