jashkenas--coffeescript/documentation/sections/breaking_change_fat_arrow.md

Bound (fat arrow) functions

In CoffeeScript 1.x, => compiled to a regular function but with references to this/@ rewritten to use the outer scope’s this, or with the inner function bound to the outer scope via .bind (hence the name “bound function”). In CoffeeScript 2, => compiles to ES2015’s =>, which behaves slightly differently. The largest difference is that in ES2015, => functions lack an arguments object:

codeFor('breaking_change_fat_arrow', 'outer(1, 2)')