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File / FileReader

Binary Data / Files: File / FileReader

What is the JavaScript File object?​

View Answer:
Interview Response: The File object in JavaScript is a representation of data from individual files. It provides info like the name, size, type, and modification date of a file.

Code Example:

const file = new File(["fileName"], "fileName.txt", {
  type: "text/plain",
});

How can you create a File object in JavaScript?​

View Answer:
Interview Response: In JavaScript, a File object can be created using its constructor, which requires an array of data parts and a filename.

Syntax: let file = new File(array, filename, options);

Code Example:

const file = new File(["fileName"], "fileName.txt", {
  type: "text/plain",
});

What's the purpose of the FileReader object in JavaScript?​

View Answer:
Interview Response: The FileReader object lets web applications asynchronously read the contents of files stored on the user's computer, using File or Blob objects.

Code Example:

function printFile(file) {
  const reader = new FileReader();
  reader.onload = (event) => {
    console.log(event.target.result);
  };
  reader.readAsText(file);
}

What are the methods provided by the FileReader object?​

View Answer:
Interview Response: The FileReader object in JavaScript provides methods like readAsArrayBuffer(), readAsDataURL(), readAsText(), and abort() to handle and manipulate file content.

What are the FileReader events?​

View Answer:
Interview Response: FileReader object fires events such as `onloadstart`, `onprogress`, `onload`, `onabort`, `onerror`, and `onloadend` during the process of reading a file.

What's the 'readAsDataURL()' method in FileReader?​

View Answer:
Interview Response: The `readAsDataURL()` method in FileReader reads the contents of the specified File or Blob object as a base64 encoded data URL string.

Code Example:

HTML

<input type="file" onchange="exposeFile()"><br>
<img src="" height="200" alt="Image preview...">

JavaScript:

function exposeFile() {
  var preview = document.querySelector('img');
  var file    = document.querySelector('input[type=file]').files[0];
  var reader  = new FileReader();

  reader.addEventListener("load", function () {
    preview.src = reader.result;
  }, false);

  if (file) {
    reader.readAsDataURL(file);
  }
}

Can you explain the 'readAsText()' method in FileReader?​

View Answer:
Interview Response: The `readAsText()` method in FileReader reads the contents of the specified File or Blob object as a text string.

Code Example:

// Create a new Blob with text content
var blob = new Blob(["Hello, world!"], {type: "text/plain"});

// Create a new FileReader
var reader = new FileReader();

// Set up a load event handler to read the Blob content as text when it's loaded
reader.onload = function(e) {
  var text = reader.result;
  console.log(text);  // Outputs: "Hello, world!"
}

// Start reading the Blob as text
reader.readAsText(blob);

In this example, when reader.readAsText(blob); is called, the reader begins reading the contents of the blob. When it's done, it triggers a "load" event, and reader.result will contain the text content of the Blob.

What is the 'onload' event in FileReader?​

View Answer:
Interview Response: The `onload` event in FileReader is triggered when the read operation successfully completes, making the file's data available through the `result` attribute.

Can you define the 'onerror' event in FileReader?​

View Answer:
Interview Response: The `onerror` event in FileReader is triggered when the read operation encounters an error, providing an error message via the `error` attribute. It can be used to handle errors during file reading operations.

What does the 'abort()' method in FileReader do?​

View Answer:
Interview Response: The `abort()` method in FileReader stops the reading process before its completion, triggering the `onabort` event.

What is the 'onabort' event in FileReader?​

View Answer:
Interview Response: The `onabort` event in FileReader is triggered when the reading process is intentionally cancelled using the `abort()` method. It's useful for handling scenarios when a read operation is intentionally cancelled.

What does the 'readyState' attribute in FileReader represent?​

View Answer:
Interview Response: The readyState attribute in FileReader represents the current state of the FileReader object, indicating the progress and status of the file reading operation. It can be EMPTY (0), LOADING (1), or DONE (2).

Code Example:

// Create a new Blob with text content
var blob = new Blob(["Hello, world!"], {type: "text/plain"});

// Create a new FileReader
var reader = new FileReader();

console.log(reader.readyState);  // Outputs: 0 (EMPTY)

// Begin reading the Blob as text
reader.readAsText(blob);

console.log(reader.readyState);  // Outputs: 1 (LOADING)

reader.onload = function() {
  console.log(reader.readyState);  // Outputs: 2 (DONE)
}

What is 'result' property in FileReader?​

View Answer:
Interview Response: The 'result' property in FileReader contains the data that has been read from the file, representing the result of a successful file reading operation. Its data type depends on the method used to initiate the read.

Code Example:

// Create a new Blob with text content
var blob = new Blob(["Hello, JavaScript!"], {type: "text/plain"});

// Create a new FileReader
var reader = new FileReader();

// Begin reading the Blob as text
reader.readAsText(blob);

reader.onload = function() {
  // The 'result' attribute contains the data read from the Blob
  console.log(reader.result);  // Outputs: "Hello, JavaScript!"
}

How is 'readAsArrayBuffer()' method in FileReader used?​

View Answer:
Interview Response: The `readAsArrayBuffer()` method in FileReader is used to read the contents of a specified File or Blob object as an ArrayBuffer, which represents binary data in a fixed-length buffer.

Code Example:

// Create a new Blob with text content
var blob = new Blob(["Hello, world!"], {type: "text/plain"});

// Create a new FileReader
var reader = new FileReader();

// Begin reading the Blob as an ArrayBuffer
reader.readAsArrayBuffer(blob);

reader.onload = function() {
  // The 'result' attribute contains the ArrayBuffer read from the Blob
  var arrayBuffer = reader.result;
  
  // Create a new DataView from the ArrayBuffer
  var dataView = new DataView(arrayBuffer);
  
  // Access the first byte in the ArrayBuffer
  var firstByte = dataView.getUint8(0);
  
  console.log(firstByte); // Outputs the Unicode value of the first character
}

What is 'readAsBinaryString()' method in FileReader?​

View Answer:
Interview Response: The `readAsBinaryString()` method is used to retrieve the contents of a file and convert them into a binary string. Each character in the string represents a single byte of data contained within the file.

Code Example:

// Create a new Blob with text content
let blob = new Blob(["Hello, JavaScript!"], {type: "text/plain"});

// Create a new FileReader
let reader = new FileReader();

// Begin reading the Blob as a binary string
reader.readAsBinaryString(blob);

reader.onload = function() {
  // The 'result' attribute contains the binary string read from the Blob
  var binaryString = reader.result;
  
  console.log(binaryString); // Outputs: "Hello, JavaScript!"
}

What is the function of the 'onprogress' event in FileReader?​

View Answer:
Interview Response: The "onprogress" event is a recurring occurrence that is triggered during the process of reading a file. Its purpose is to provide updates on the progress of the read operation, allowing the user to keep track of how much has been completed thus far.

Code Example:

// Create a new Blob with large content
let largeText = 'a'.repeat(1000000); // 1 million characters
let blob = new Blob([largeText], {type: "text/plain"});

// Create a new FileReader
let reader = new FileReader();

reader.onprogress = function(e) {
  if (e.lengthComputable) {
    let percentLoaded = Math.round((e.loaded / e.total) * 100);
    console.log(`Progress: ${percentLoaded}%`);
  }
};

// Begin reading the Blob as text
reader.readAsText(blob);

What is the 'onloadstart' event in FileReader?​

View Answer:
Interview Response: To set up tasks before a read operation starts, you use the `onloadstart` event. This event is triggered just before the read process is initiated, allowing for necessary preparation.

What is the 'onloadend' event in FileReader?​

View Answer:
Interview Response: The 'onloadend' event in FileReader is triggered when the reading operation is completed, whether successful or not. It allows the application to take appropriate action based on the outcome of the operation.

How does the FileReader handle large files?​

View Answer:
Interview Response: The FileReader API is not suitable for handling large files as it loads the entire file into memory, causing performance issues. Other alternatives like ReadableStream API or using server-side processing should be considered.

Code Example: Chunking FileReader

var blob = new Blob(["Very large file content..."], {type: "text/plain"}); // Assume a large Blob here

var chunkSize = 1024; // Size of each chunk
var start = 0; // Start position for each read operation
var end = chunkSize; // End position for each read operation

// Create a new FileReader
var reader = new FileReader();

// Event handler for read operation
reader.onload = function(e) {
  // Handle each chunk of data here
  var chunk = reader.result;
  console.log(chunk);

  // Update the start and end position for next chunk
  start += chunkSize;
  end = start + chunkSize;

  // Read the next chunk
  if (start < blob.size) {
    readNextChunk();
  }
};

// Function to read next chunk
function readNextChunk() {
  var chunk = blob.slice(start, end);
  reader.readAsText(chunk);
}

// Start reading the first chunk
readNextChunk();

In this example, we're reading a large Blob in chunks of size 1024 bytes. After each chunk is read, we move on to the next one until we've read the entire Blob. This method allows the FileReader to handle very large files efficiently.

tip

Other alternatives like ReadableStream API or using server-side processing should be considered.

Can you explain the function of the File Object/Constructor?​

View Answer:
Interview Response: The File object constructor in programming creates a file instance representing a file in the file system, providing methods to read, write, and manipulate the file.

Technical Response: A File object inherits from Blob and gets extended with filesystem-related capabilities. A File object is a specific kind of a Blob and can be used in any context that a Blob can. In particular, FileReader, URL.createObjectURL(), createImageBitmap(), and XMLHttpRequest.send() which accepts both Blobs and Files. The File() constructor creates a new File object instance. The File constructor has three arguments: fileParts(bits), filename, and optional parameters. The fileParts can include an Array of ArrayBuffer, ArrayBufferView, Blob, USVString objects, or a mix of any of such objects put inside the File. USVString objects get encoded as UTF-8. The filename is a USVString filename, and the optional options parameter is lastModified, which is the timestamp of the last modification.

Code Example:

Syntax: new File(fileParts, fileName, [options]);

var file = new File(['foo'], 'foo.txt', {
  type: 'text/plain',
});

Can you explain how the FileReader Object functions?​

View Answer:
Interview Response: The FileReader object allows web applications to asynchronously read contents of files stored on the user's computer, using File or Blob objects to specify the file or data to read.

Technical Response: The FileReader object allows web applications to asynchronously read the contents of files (or raw data buffers) saved on the user's machine by specifying the file or data to read with File or Blob objects. We can retrieve File objects via a FileList object returned by a user choosing files using the ‹input› element, from the DataTransfer object of a drag and drop operations, or through the mozGetAsFile() API on an HTMLCanvasElement. FileReader can only read the contents of files that the user has expressly selected, either through an HTML ‹input type="file"› element or by drag and drop. However, We cannot use it to read a file by pathname from the user's file system. We should note that OS level access is restricted in JavaScript.

Code Example:

Syntax: let reader = new FileReader(); // no arguments

<input type="file" onchange="readFile(this)" />

<script>
  function readFile(input) {
    let file = input.files[0];

    let reader = new FileReader();

    reader.readAsText(file);

    reader.onload = function () {
      console.log(reader.result);
    };

    reader.onerror = function () {
      console.log(reader.error);
    };
  }
</script>