I am trying to understand more on how callback functions work in JavaScript.
I found this simple example code online and after adding the imports & additional txt file it's working. However I am not sure why it is working as the parameters of the functions aren't being defined anywhere?
fs.readFile(`${__dirname}/temp.txt`, (err, data) => {
console.log(`Breed: ${data}`);
superagent.get(`https://dog.ceo/api/breed/${data}/images/random`).end((err,res) =>{
console.log(res.body)
}); })
Upon running this I get the expected result on the console with the line displaying Breed: ${data} and the next line showing the body of the res.
However, the err, data parameters have never been provided to the arrow function within readFile. Neither has the err,res parameters ever been called/provided a value for it to display a message to the console, then how are these lines displaying a message to the console?
I could have understood this working if .readFile returned a err and data value & .end returned a err and res value, however upon reading the documentation both these functions return void, thus i'm confused and help would be appreciated.
You pass a function in to readFile, and later on readFile will call your function. When it calls your function, readFile will pass in the relevant values.
You can do something similar yourself, like this:
function myReadFile(filename, callback) {
setTimeout(() => {
if (Math.random() > 0.5) {
callback(null, 'fake file');
} else {
callback('an error', null);
}
}, 1000);
}
myReadFile(`${__dirname}/temp.txt`, (err, data) => {
console.log(`Breed: ${data}`);
})
(though of course the real readFile function isn't doing timeouts or random numbers, but rather reading from a file and listening for that operation to succeed or error out)