What is Fat comma?

The fat comma is a syntactic construction that appears in a position in a function call where a comma would usually appear. The original usage refers to the "]letters:[" construction in ALGOL 60. Newer usage refers to the "=>" operator present in some programming languages. It is primarily associated with PHP, Ruby and Perl programming languages, which use it to declare hashes. Using a fat comma to bind key-value pairs in a hash, instead of using a comma, is considered an example of good idiomatic Perl. In CoffeeScript and TypeScript, the fat comma is used to declare a function that is bound to this.