module.export behave oddly

Question

There is a surprising behaviour with module.exports . The following code:

module.exports = { test: 4};
module.exports.testMe = () => {console.log(this.test);};

Will output undefined . But:

module.exports.test = 4;
module.exports.testMe = () => {console.log(this.test);};

Will output 4 . Why does it behave differently?

Answer 1

The reason has to do with how arrow functions work, they bind to this (much like if you had used .testMe = fn.bind(this); ).

In the first case, you're overwriting module.exports , but this still points to the old object. So when testMe() is implicitly bound to this , you get undefined because there was no test property on the original module.exports .

In the second case, you're not overwriting module.exports , so module.exports === this and testMe() is implicitly bound to this and outputs '4' as expected because both point to the same object, where test exists.