Expressjs middleware keeps variable changed

Question

I am trying to do a simple thing which is obvious I believe in the code below:

module.exports = function(req, res, next) {
    var dop = require('../../config/config').DefaultOptions;
    console.log(require('../../config/config').DefaultOptions);
    console.log(dop);
    dop.firstPage = 'test.sjs';
    next();
};

This is an Expressjs middle ware which is very simple but the interesting point is that next time I load a page both of the console.log results has been changed to 'firstPage: test.sjs'. It shouldn't act like this and it should only change the dop variable.

Anyone with the knowledge why this creepy thing is happening?

Thank you

assuming the value concerned is an object, then `dop` becomes a reference to that same object, and any change you make to it affects the original one as well — Robin Zigmond, Mar 01 '19 at 15:47
Is there anyway to avoid it? Like making ```dop``` standalone. — Shadow4Kill, Mar 01 '19 at 15:49
yes, see [this question](https://stackoverflow.com/questions/728360/how-do-i-correctly-clone-a-javascript-object). Although your `dop` is a local variable only so whatever you're trying to do by setting it, you won't then be able to observe outside of this function. — Robin Zigmond, Mar 01 '19 at 15:51
Thank you. Yes ```dop``` is local here because I simplified it to post it here. — Shadow4Kill, Mar 01 '19 at 15:53

score 0 · Answer 1 · answered Mar 01 '19 at 16:20

The main issue is require() is cached, so require('../../config/config') returns reference to the same instance, and as a result changing in one place causes all other references and subsequent requires to get that modified instance.

The simplest solution would be have a function in config to return a config object, that way every time invoking the get config function you will get a new instance with essentially the same content. I.e.:

config.js:

module.exports = {
  getDefaultOptions: function(){
    return {foo: 'bar', ip: '1.1.1.1'}
  }
};

Expressjs middleware keeps variable changed

1 Answers1