Get data from fs.readFile


Question

var content;
fs.readFile('./Index.html', function read(err, data) {
    if (err) {
        throw err;
    }
    content = data;
});
console.log(content);

Logs undefined, why?

1
266
11/27/2014 9:11:26 AM

Accepted Answer

To elaborate on what @Raynos said, the function you have defined is an asynchronous callback. It doesn't execute right away, rather it executes when the file loading has completed. When you call readFile, control is returned immediately and the next line of code is executed. So when you call console.log, your callback has not yet been invoked, and this content has not yet been set. Welcome to asynchronous programming.

Example approaches

const fs = require('fs');
var content;
// First I want to read the file
fs.readFile('./Index.html', function read(err, data) {
    if (err) {
        throw err;
    }
    content = data;

    // Invoke the next step here however you like
    console.log(content);   // Put all of the code here (not the best solution)
    processFile();          // Or put the next step in a function and invoke it
});

function processFile() {
    console.log(content);
}

Or better yet, as Raynos example shows, wrap your call in a function and pass in your own callbacks. (Apparently this is better practice) I think getting into the habit of wrapping your async calls in function that takes a callback will save you a lot of trouble and messy code.

function doSomething (callback) {
    // any async callback invokes callback with response
}

doSomething (function doSomethingAfter(err, result) {
    // process the async result
});
309
8/26/2018 11:00:34 AM

There is actually a Synchronous function for this:

http://nodejs.org/api/fs.html#fs_fs_readfilesync_filename_encoding

Asynchronous

fs.readFile(filename, [encoding], [callback])

Asynchronously reads the entire contents of a file. Example:

fs.readFile('/etc/passwd', function (err, data) {
  if (err) throw err;
  console.log(data);
});

The callback is passed two arguments (err, data), where data is the contents of the file.

If no encoding is specified, then the raw buffer is returned.


SYNCHRONOUS

fs.readFileSync(filename, [encoding])

Synchronous version of fs.readFile. Returns the contents of the file named filename.

If encoding is specified then this function returns a string. Otherwise it returns a buffer.

var text = fs.readFileSync('test.md','utf8')
console.log (text)

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