The example below is a function that returns a value using a callback function at stream completion. After reading the file, the end callback function returns a value. However, totalBytes is undefined. This is because return totalBytes is a value returned from the callback function of the end event.
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const fs = require('fs');
function getTotalBytes(filePath) {
let totalBytes = 0;
const readStream = fs.createReadStream(filePath);
readStream.on('data', (chunk) => {
totalBytes += chunk.length;
});
readStream.on('end', () => {
return totalBytes;
});
}
const filePath = 'example.txt';
const totalBytes = getTotalBytes(filePath);
console.log(totalBytes); // Output: undefined
Solution
If you modify it to return the Promise object, you can get the desired value.
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const fs = require('fs');
function getTotalBytes(filePath) {
return new Promise((resolve, reject) => {
let totalBytes = 0;
const readStream = fs.createReadStream(filePath);
readStream.on('data', (chunk) => {
totalBytes += chunk.length;
});
readStream.on('end', () => {
resolve(totalBytes);
});
readStream.on('error', (error) => {
reject(error);
});
});
}
const filePath = 'example.txt';
getTotalBytes(filePath)
.then((totalBytes) => console.log(totalBytes))
.catch((error) => console.error(error));