Why does `var exports = module.exports = {};` works but not `let exports = module.exports = {};`?

During code optimisation of my project, I was replacing all instances of var keyword with let because I think there is no particular use left for var. I did a 'Find & Replace' in all files to do this.

In some custom modules I had used statements like:

var exports = module.exports = {};

which worked fine.

Now, after replacing it with let, it became:

let exports = module.exports = {};

It does not work now and gives an error

SyntaxError: Identifier 'exports' has already been declared

Though I can use var and avoid this problem, I still want to know what is the reason behind this.

via Mohit Bhardwaj