Access function in module from another function

Question

Right now I have a module with two methods. How can I access downloadFile function to be reused in the downloadFiles function. Right now it will throw exception that the downloadFile is not defined. Thanks in advance.

exports.downloadLib = {
    downloadFile: async function (fileUrl, dest) {
        const shell = require('node-powershell');

        let ps = new shell({
            executionPolicy: 'Bypass',
            noProfile: true
        });

        let commandString = `iwr ${fileUrl} -OutFile ${dest}`;
        ps.addCommand(commandString);

        try {
            await ps.invoke();
        } catch (e) {
            console.log(`ERROR - ${e}`);
        } finally {
            await ps.dispose();
            console.log(`finished download file ${dest}`)
        }
    },

    downloadFiles: function (fileUrls) {
        fileUrls.forEach(function (fileUrl) {
            downloadFile(fileUrl, fileUrl.substring(fileUrl.lastIndexOf('/') + 1))
        }, this);
    }
}

this.downloadFile

Jonas Wilms
– Jonas Wilms

2017-04-17 15:36:10 +00:00
Commented Apr 17, 2017 at 15:36 — Jonas Wilms
– Jonas Wilms, Commented Apr 17, 2017 at 15:36
You can use this.downloadFile inside downloadFiles.

Mauro Bringolf
– Mauro Bringolf

2017-04-17 15:37:09 +00:00
Commented Apr 17, 2017 at 15:37 — Mauro Bringolf
– Mauro Bringolf, Commented Apr 17, 2017 at 15:37

John Vandivier · Accepted Answer · 2017-04-17 15:37:17Z

1

Create a private method for the module. Separately, inject this into the module and also invoke it where you need to. The underscore prefix is a convention used by many to denote a private method.

async function _downloadFile(fileUrl, dest) {
  const shell = require('node-powershell');

  let ps = new shell({
    executionPolicy: 'Bypass',
    noProfile: true
  });

  let commandString = `iwr ${fileUrl} -OutFile ${dest}`;
  ps.addCommand(commandString);

  try {
    await ps.invoke();
  } catch (e) {
    console.log(`ERROR - ${e}`);
  } finally {
    await ps.dispose();
    console.log(`finished download file ${dest}`)
  }
}

exports.downloadLib = {
  downloadFile: _downloadFile,

  downloadFiles: function(fileUrls) {
    fileUrls.forEach(function(fileUrl) {
      _downloadFile(fileUrl, fileUrl.substring(fileUrl.lastIndexOf('/') + 1))
    }, this);
  }
}

You can also keep _downloadFile() inside the module and do something like this._downloadFile() or context._downloadFile(). You will need to define let context = this at the top of the module for the latter.

answered Apr 17, 2017 at 15:37

John Vandivier

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2 Comments

John Vandivier Over a year ago

easy to change whether or not you export _downloadFile

John Vandivier Over a year ago

Let me admit your answer is simpler. I just gave the answer I knew worked, not arguing it's perfect. Hm also mine avoids this in case a developer has an aversion to it.

Jonas Wilms · Accepted Answer · 2017-04-17 15:37:48Z

0

downloadFiles: function (fileUrls) {
    fileUrls.forEach(function (fileUrl) {
        this.downloadFile(fileUrl, fileUrl.substring(fileUrl.lastIndexOf('/') + 1))
    }, this);
}

downloadFile is not part of downloadFiles Scope, but of its context.

answered Apr 17, 2017 at 15:37

Jonas Wilms

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5 Comments

Yordan Kanchelov Over a year ago

This ain't proper in this way. The context of this in that way is the global object.

Jonas Wilms Over a year ago

@Jordan Kanchelov it is. Looks like you dont know about forEachs second param... Hint: this is not global...

Yordan Kanchelov Over a year ago

If I use 'use strict' "this" is undefined in this case. In this case when I am not using it should point to the global ?

Jonas Wilms Over a year ago

@JordanKanchelov NOPE!!! have a look at Function.prototype.call. Thats what forEach is doing. This is passed into the forEach loop

Jonas Wilms Over a year ago

@JordanKanchelov developer.mozilla.org/en/docs/Web/JavaScript/Reference/…

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