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readFile. Returns the contents of the file named filename. If encoding is specified then this function returns a string. Otherwise it returns a buffer.
readFile() method, we can read a file in a non-blocking asynchronous way, but in fs. readFileSync() method, we can read files in a synchronous way, i.e. we are telling node. js to block other parallel process and do the current file reading process.
Syntax: fsPromises.readFile( path, options ) Parameters: The method accept two parameters as mentioned above and described below: path: It holds the name of the file to read or the entire path if stored at other location. It is a string, buffer, URL or a filename. options: It holds the encoding of file.
readFileSync() is synchronous and blocks execution until finished. These return their results as return values. readFile() are asynchronous and return immediately while they function in the background. You pass a callback function which gets called when they finish.
The following piece of code creates a text file and then reads it, overwrites it, and reads it again. Except the creation of the file the three I/O operations are performed using Node.js async readFile and writeFile.
I don't understand why the first read is returning no error but no data either. The output of this code is:
Even if the operations were to happen in an arbitrary order (due to their async nature) I would have NOT expected to get an "empty data" object.
Any ideas why I am getting an empty data when reading the file (and no error) ?
Is there anything that I can do to make sure the file content is read?
var fs = require('fs');
var fileName = __dirname + '/test.txt';
// Create the test file (this is sync on purpose)
fs.writeFileSync(fileName, 'initial test text', 'utf8');
console.log("Starting...");
// Read async
fs.readFile(fileName, 'utf8', function(err, data) {
var msg = "";
if(err)
console.log("first read returned error: ", err);
else {
if (data === null)
console.log("first read returned NULL data!");
else if (data === "")
console.log("first read returned EMPTY data!");
else
console.log("first read returned data: ", data);
}
});
// Write async
fs.writeFile(fileName, 'updated text', 'utf8', function(err) {
var msg = "";
if(err)
console.log("write finished with error: ", err);
else
console.log("write finished OK");
});
// Read async
fs.readFile(fileName, 'utf8', function(err, data) {
var msg = "";
if(err)
console.log("second read returned error: ", err);
else
if (data === null)
console.log("second read returned NULL data!");
else if (data === "")
console.log("second read returned EMPTY data!");
else
console.log("second read returned data: ", data);
});
console.log("Done.");
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