Question

Working on a NodeJS project, I came a across this very unexpected behaviour that I can’t figure a way around – it seems like a bug to me, but perhaps I’m simply misunderstanding how NodeJS modules operate.

I’ve reduced it into a testcase as follows:

mod.js module

exports.process = function(obj) { obj.two = 'two'; };

test.js file

var testObj = {one: 'one'};


console.log(['Before:', testObj]);

var cachedObj = testObj;
require('./mod').process(cachedObj);

console.log(['After:', testObj]);

Then running $ node test.js gives me this:

[ 'Before:', { one: 'one' } ]
[ 'After:', { one: 'one', two: 'two' } ]

I’m assigning the value of testObj to cachedObj, and testObj is never being passed to the module method. testObj should (as far as I can see) never be modified at all.

In fact, cachedObj should surely never be modified either, as it is never returned from the mod.process method. Where am I going wrong?

(running Node 0.6.9)

Answer 1

It’s not a bug, it’s perfectly expected behavior.

Variables in JavaScript are passed by reference, so the original object is mutated by the assignment in process.