This way the file can be downloaded or shown up as an image, as a part of canvas etc.Īnd if we’re going to send a File over a network, that’s also easy: network API like XMLHttpRequest or fetch natively accepts File objects. Just as we did with blobs, we can create a short url with URL.createObjectURL(file) and assign it to or. In many cases though, we don’t have to read the file contents. Data url, base-64 encoded ( readAsDataURL).We usually get File objects from user input, like or Drag’n’Drop events ( ondragend).įileReader objects can read from a file or a blob, in one of three formats: In addition to Blob methods and properties, File objects also have name and lastModified properties, plus the internal ability to read from filesystem. That’s only inside a Web Worker though, because delays in synchronous calls, that are possible while reading from files, in Web Workers are less important. Its reading methods read* do not generate events, but rather return a result, as regular functions do. The most widely used events are for sure load and error.įor Web Workers, there also exists a synchronous variant of FileReader, called FileReaderSync. reader.result is the result (if successful).When the reading is finished, we can access the result as: When a file is selected by using the input typefile object, the value of the value property depends on the value of the 'Include local directory path when uploading files to a server' security setting for the security zone used to display the Web page containing the input object. loadend – reading finished with either success or failure.There’s an alternative to reading a file for that, as discussed in chapter Blob: URL.createObjectURL(file).Īs the reading proceeds, there are events: readAsDataURL – when we’d like to use this data in src for img or another tag.readAsText – for text files, when we’d like to get a string. For high-level operations, like slicing, File inherits from Blob, so we can call them directly, without reading.
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