Undefined 'this' in Promise + array. reduce()

Question

I'm facing a problem since this morning.

WHAT I HAVE

Basically, I have a simple class, with an array of files to register:

function MyClass() {

    this.filesToRegister = [
        {
            "fileName": "http://tny.im/azk"
        },
        {
            "fileName": "http://tny.im/azk"
        }
    ];
}

I have also a simple function, _contextFunction() which takes a single fileToRegister entry:

MyClass.prototype._contextFunction = function(fileToRegister) {

    return new Promise((resolve, reject) => {

        logger.info(typeof this.filesToRegister);
        logger.info('current file: ' + fileToRegister);
        return resolve();
    });
};

Note that this function MUST access to the context (the this), it's mandatory, and I can't change that.

Finally, I have a utility method, processArray(), that can apply a function on each item of an array, all done synchronously:

MyClass.prototype.processArray = function(array, fn) {

    let results = [];
    return array.reduce((p, item) => {

        return p.then(() => {

            return fn(item).then((data) => {

                results.push(data);
                return results;
            }).catch(err => console.log(err));
        });
    }, Promise.resolve());
};

WHAT I TRY TO DO

I use this utility method to apply _contextFunction() on each item of the filesToRegister array:

this.processArray(this.filesToRegister, this._contextFunction);

It works without problem and execute this._contextFunction() on each item of this.filesToRegister.

WHAT THE PROBLEM IS

BUT, when I try to log typeof this.filesToRegister in _contextFunction(), the result is undefined... After several tests, I concluded that nothing in the context is accessible (neither context attributes nor context methods).

However, if I execute this._contextFunction() without the processArray() method, I can access to the context (both context attributes and context methods).

WHAT I THINK

My guess is that the problem comes from the processArray() method, but I don't see where... I tried to log typeof this.filesToRegister right in the processArray() method, and it works...

To conclude:

• processArray() IS able to access to the context.
• this._contextFunction() launched 'standalone' IS able to access to the context.
• this._contextFunction() launched by processArray() IS NOT able to access to the context.

Can anyone help me? Thanks

Answer 1

fn(item)

Calls the function without the context. Use:

fn.call(this, item)

---

Alternatively pass the method name:

this.processArray(this.filesToRegister,"_contextFunction");

And then do:

this[fn](item);

---

How i would do that:

class MyClass {
 constructor(){
   this.files = [];
 }
 async add(file){
   await "whatever";
   this.files.push(file);
 }
 multiple(name, array){
   return Promise.all( array.map(el => this[name](el)));
 }
}

And then:

const instance = new MyClass;
instance.add("whatever");
instance.multiple("add",[1,2,3]);