Can someone walk me through this es6 code?

This code does something rather silly. Here's the rundown:

'use strict'

This just means that variables must be explicitly declared. Globals will not be created by default.

var invoice = {
  number: 123,
  process: function() {
    return () => console.log(this.number);
  }
};

This creates an object named invoice, with a property named number (set to the value 123) and a method named process. The process method returns an anonymous function which will write the value of invoice.number to the log.

var newInvoice = {
  number: 456
}

This creates an object named newInvoice, with a property named number (set to the value 456).

invoice.process().bind(newInvoice)();

Now we get to the silly part.

Here, invoice.process() returns that anonymous function, and .bind creates a new function that will use newinvoice as its "this" (instead of invoice). The final set of parentheses cause the new function to be invoked.

What makes this silly is that the function returned by invoice.process() has already resolved "this" when it was created and no longer cares what "this" refers to. So this whole line is equivalent to "(invoice.process())()" which itself is equivalent to "console.log(invoice.number)".

Does it make sense now? And where did you find this?