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ViewsWhy does this module return undefined?
I'm writting a code in NodeJs using jimp and qrcode-reader. Those dependencies work well. The issue lies when I export the code into a module. And supposedly, it should return a value. This is an arrow function that returns the URL of the QR Code. However, If I use "console.log", I can see it works. But when I use the module, it returns undefined. Perhaps I'm missing something real simple.
Here is the code of the module:
const Jimp = require('jimp')
const fs = require('fs')
const QrCode = require('qrcode-reader')
const scanQR = (source) => {
const buffer = fs.readFileSync(source)
Jimp.read(buffer, (err, image) => {
try {
const qr = new QrCode()
qr.callback = (error, value) => {
try {
console.log(value.result)
return value.result
} catch(error) {
console.error(error)
}
}
qr.decode(image.bitmap)
} catch (error) {
console.error(error)
}
})
}
module.exports = scanQR
Here is the module imported:
const scanQR = require('./utils/scanQR')
const qr_code_screenshot = `${__dirname}/assets/images/qrcode-test.png`
const result = scanQR(qr_code_screenshot)
console.log(result)
Here is the result on console:
Thanks in advance.
1 answers
Answer question0
Your scanQR function is not returning anything: and therefore when you assign its return value, you get undefined. That is expected. The issues comes from you attempting to returning something inside a callback that is only fired asynchronously.
To solve this issue you will need to wrap the inner logic in a promise (which is returned). Then, you can resolve/reject the promise based on whether the code was successfully executed, or threw an error:
const scanQR = (source) => {
// Wrap all logic inside a promise, which you return
return new Promise((resolve, reject) => {
const buffer = fs.readFileSync(source);
Jimp.read(buffer, (err, image) => {
try {
const qr = new QrCode();
qr.callback = (error, value) => {
try {
// Resolve promise here
resolve(value.result);
} catch(error) {
// Reject promise
reject(error);
}
}
qr.decode(image.bitmap);
} catch (error) {
reject(error);
}
});
});
}
When consuming the module, you will need to either use .then():
const scanQR = require('./utils/scanQR');
const qr_code_screenshot = `${__dirname}/assets/images/qrcode-test.png`;
// `scanQR` returns a promise and you will need to use `.then()`
scanQR(qr_code_screenshot).then(data => console.log(data));
...or use async/await: here is one of the many ways to do it:
const scanQR = require('./utils/scanQR');
async function scan(image) {
const result = await scanQR(image);
console.log(result);
}
scan(`${__dirname}/assets/images/qrcode-test.png`);