作为函数参数传递后，回调将变为null

Question

I have 2 functions funcA and funcB . funcA calls funcB and passes a callback. In funcA the callback ( cb ) is well-defined, but funcB gets undefined .

Here is the code:

MyClass.prototype.funcA= function (a) {
    return new Promise((resolve, reject) => {
        let cb = (reading) => {
            console.log("Will resolve:" + reading);
            resolve(funcC(reading));
        };

        if (cb) 
            console.log("cb ok");
        else
            console.log("cb ko");

        this.funcB(a, cb);
    });
}

MyClass.prototype.funcB= function (a, callback, c) {
    console.log("funcB");
    console.log("callback: " + callback);
    this.device.send(a, response => {
        console.log("CB ?");
        if (callback) {
            console.log("CB !");
            callback(response);
        }
        else {
            console.log("no callback");
            console.log(callback);
        }
    });

    if (c) {
        funcB([0x00, 0x00]);
    }
};

And here is the console output:

cb ok
funcB
callback: (reading) => {
            console.log("Will resolve:" + reading);
            resolve(funcC(reading));
        }
CB ?
no callback
undefined

Note that in the code base I have other functions that calls funcB and funcB successfully calls the callback.

Answer 1

I've found the issue.

this.device.send() does not call the callback it is given, but the callback given in a previous call. As it was not documented I had to go 3 layers down to find the implementation...

So the solution is to call funcB() until the callback you want is called.