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--- java-script:read-file [2023/08/04 23:17] – [How to read a file (both web and node.js)] odefta
+++ java-script:read-file [2023/08/20 01:19] (current) – ↷ Links adapted because of a move operation 217.113.194.57
@@ Line 3: / Line 3: @@
 ===== Read a file in web browser =====
-The following script reads the content of a file selected by the user and then displays those contents in a <pre> element. \\ When the "Read File" button is clicked, the readFile function is called, which reads the file selected in the file input field and writes its contents to the fileContents element.
+The following script reads the content of a file selected by the user and then displays those contents in a **<pre>** element. \\ When the "Read File" button is clicked, the **readFile** function is called, which reads the file selected in the file input field and writes its contents to the **fileContents** element.
+<note>
+The read file operation using the FileReader API in a web browser is **asynchronous**. When you call readAsText (or any of the other read methods on a FileReader object), it starts the read operation and then immediately returns. The actual reading of the file happens in the background, and when it's done, the onload event is fired. \\ \\
+This is why you provide** an onload callback function (or a promise)** to handle the file data once it's available. If this operation were synchronous (i.e., blocking), the browser would become unresponsive until the read operation completes, which would lead to a poor user experience, especially for large files that might take a noticeable amount of time to read. \\ \\
+In the case of** Node.js and the fs module**, you have the option to read files synchronously using readFileSync, but it's generally recommended to use the asynchronous readFile for the reasons mentioned above. Asynchronous I/O operations allow the Node.js event loop to continue processing other tasks while waiting for the I/O operation to complete, leading to better performance and scalability.
+</note>
 <note important>
@@ Line 17: / Line 25: @@
 </note>
+==== Using callback function (on onload event) ====
+The **FileReader** object's **onload** event uses a callback function to handle the event when the file's content is fully loaded into memory.
+<code javascript read-file-web-callback.html>
+<!DOCTYPE html>
+<html>
+<head>
+    <title>File Reader Example</title>
+    <script>
+        function readFile() {
+            const fileInput = document.getElementById('myFile');
+            const file = fileInput.files[0];
+            const reader = new FileReader();
+            reader.onload = function(e) {
+                const contents = e.target.result;
+                document.getElementById('fileContents').innerText = contents;
+            };
+            reader.readAsText(file);
+        }
+    </script>
+</head>
+<body>
+    <input type="file" id="myFile">
+    <button onclick="readFile()">Read File</button>
+    <pre id="fileContents"></pre>
+</body>
+</html>
+</code>
+==== Using Promise ====
+If you prefer using **Promise** instead of the **onload** event, you can create a wrapper function around the FileReader API that returns a promise. \\
+A new function **readAsText** wraps the FileReader API and returns a promise. This promise resolves with the file's contents when the onload event is fired, and rejects if there's an error during the read operation (onerror). \\ Now the readFile function uses this readAsText function and handles the file content in a then clause, which gives a more modern, promise-based way of handling the file reading operation.
+<code javascript read-file-web-promise.html>
+<!DOCTYPE html>
+<html>
+<head>
+    <title>File Reader Example</title>
+    <script>
+        function readFile() {
+            const fileInput = document.getElementById('myFile');
+            const file = fileInput.files[0];
+            readAsText(file).then(contents => {
+                document.getElementById('fileContents').innerText = contents;
+            });
+        }
+        function readAsText(file) {
+            return new Promise((resolve, reject) => {
+                const reader = new FileReader();
+                reader.onload = function(e) {
+                    resolve(e.target.result);
+                };
+                reader.onerror = function(e) {
+                    reject(e);
+                };
+                reader.readAsText(file);
+            });
+        }
+    </script>
+</head>
+<body>
+    <input type="file" id="myFile">
+    <button onclick="readFile()">Read File</button>
+    <pre id="fileContents"></pre>
+</body>
+</html>
+</code>
+===== Read a file in Node.js =====
+First you should [[java-script:node-js:setup|initialize the directory as a node.js es6+ module.]]
+<code javascript fileReader.js>
+import fs from 'fs/promises';
+export async function readFile(filePath) {
+    try {
+        const data = await fs.readFile(filePath, 'utf8');
+        console.log('File content:', data);
+    } catch (err) {
+        console.error('An error occurred:', err);
+    }
+}
+</code>
+<code javascript index.js>
+import { readFile } from './fileReader.js';
+const fileName = process.argv[2];
+if (!fileName) {
+    console.log('Please provide a file name as a command-line argument');
+    process.exit(1);
+}
+readFile(fileName);
+</code>
+To run it:
+<code>
+node ./index.js C:/test.txt
+</code>