Why Can't My JavaScript Code Read Local Text Files, and How Can I Fix It?

Troubleshooting Local Text File Reading in the Browser

In an attempt to create a basic text file reader, a JavaScript function was developed to take in a file path, parse each line into a character array, but encountered errors. This article aims to diagnose and resolve these issues.

Error Diagnosis

Initially, the code failed to display the text content, despite successfully retrieving the file's path. Upon debugging, it was discovered that the XMLHttpRequest object was not correctly handling local file access.

Changing to Firefox yielded successful results, highlighting a potential browser-specific issue. Chrome, in particular, was throwing an XMLHttpRequest exception 101, indicating a network error.

Solution for Browser Compatibility

To address the browser compatibility issue, the Fetch API, introduced in JS in 2015, offers a simpler and more robust approach to fetching data from URLs. Here's an updated version of the code:

fetch("myText.txt")
  .then((res) => res.text())
  .then((text) => {
    // process the text content
  })
  .catch((e) => console.error(e));

Accessing Local Files

Modern browsers severely restrict direct filesystem access, so using file:/// should be avoided. Instead, lightweight webservers such as python -m http.server or npx http-server can be employed to load data via standard HTTP URLs.

Handling Status 0

When accessing local files using XMLHttpRequest, no status is returned. To resolve this, a check for status 0 is necessary:

function readTextFile(file) {
  var rawFile = new XMLHttpRequest();
  rawFile.open("GET", file, false);
  rawFile.onreadystatechange = function () {
    if (rawFile.readyState === 4) {
      if (rawFile.status === 200 || rawFile.status == 0) {
        // process the text content
      }
    }
  };
  rawFile.send(null);
}

Example Usage

To use the updated code, specify the file path using file:///:

readTextFile("file:///C:/your/path/to/file.txt");

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