Cannot read property of undefined in class

Question

I'm new to javascript so can anyone help me figure out why this code is not working?

I have a class and it calls a cordova barcode scanning function. I've got an example that works, however I want to be able to separate out the function(result) and function(error) and use onSuccess(result) and onFailure(error).

I have no idea why this is happening so if anyone can help that would be great.

EDIT: so ive updated the code based on Stradosphere said however im still getting result is not defined errors.

Full error message:

Uncaught ReferenceError: result is not defined at barcodeScanner.scanBarcode (barcodeScanner.js:10) at HTMLButtonElement.myFunction (main.js:18)

var me = this;
class barcodeScanner {
    constructor() {
        this._barcodeResult = 0;
    }

    scanBarcode() {
        //THIS THROWS result is not defined error 
        cordova.plugins.barcodeScanner.scan(me.onSuccess(result), me.onFailure(error));

        //THIS WORKS
        cordova.plugins.barcodeScanner.scan(
            function (result) {
                me._barcodeResult = result.text;
                alert("Barcode Scanned:" + me._barcodeResult);        
            },
            function (error) {
                alert("Scanning failed: " + error);
            }
        );
    }

    onSuccess(result) {
        this._barcodeResult = result.text;
        alert("Barcode Scanned:" + this._barcodeResult);
    }

    onFailure(error) {
        alert("Scanning failed: " + error);
    }
}

Possible duplicate of [How to access the correct \`this\` inside a callback?](https://stackoverflow.com/questions/20279484/how-to-access-the-correct-this-inside-a-callback) — CRice, Apr 24 '18 at 22:24
CRice hi i tried implementing the answer in that thread but i didnt seem to work. ive updated the code based on another answer but that still doesnt work. So if you have any ideas whats going wrong i would happy to hear them. — Zyie, Apr 24 '18 at 23:01

Mark · Accepted Answer · 2018-04-24T23:18:57.613

1

Looking at the docs, it appears that cordova.plugins.barcodeScanner.scan() expects you to pass a function into it. But you are calling it like this:

 cordova.plugins.barcodeScanner.scan(me.onSuccess(result), me.onFailure(error));

This is passing the result of the function .onSuccess(result), but result is not defined, so you are getting an error. Additionally, you want this to be the class instance, but by defining me as this outside the class, me won't equal the class instance like you want it to. But you don't need it anyway.

Try passing functions in instead:

cordova.plugins.barcodeScanner.scan((result) => this.onSuccess(result),(error)=> this.onFailure(error))

edited Apr 24 '18 at 23:18

answered Apr 24 '18 at 23:07

Mark

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hi im now getting this error: uncaught TypeError: me.onFailure is not a function – Zyie Apr 24 '18 at 23:11
@Zyie Don't use `me`. Use `this.onFailure ()`. – Mark Apr 24 '18 at 23:12
Ok that worked. Just one slight change the onFailure takes an error youve given it a result. Thank you for your help – Zyie Apr 24 '18 at 23:16

score 0 · Answer 2 · answered Apr 24 '18 at 22:19

0

Maybe a scope issue on your use of this. Try:

var me = this; //(put this at class level)
cordova.plugins.barcodeScanner.scan(me.onSuccess, me.onFailure);

answered Apr 24 '18 at 22:19

Stradosphere

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i put the var me = this inside the constructor and i now get this error: Uncaught ReferenceError: me is not defined at barcodeScanner.scanBarcode (barcodeScanner.js:10) at HTMLButtonElement.myFunction (main.js:18) – Zyie Apr 24 '18 at 22:39

Cannot read property of undefined in class

2 Answers2