Fat comma (also termed hash rocket in Ruby and a fat arrow in JavaScript) 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.
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Perl
The "fat comma" forces the word to its left to be interpreted as a string.
Thus, where this would produce a run-time error under strict (barewords are not allowed):
the following use of the fat comma would be legal and idiomatic:
This is because the token converted_to_string would be converted to the string literal "converted_to_string" which is a legal argument in a hash key assignment. The result is easier-to-read code, with a stronger emphasis on the name-value pairing of associative arrays.
PHP
In PHP, the fat comma is termed a double arrow, and is used to specify key/value relationships when declaring an array. Unlike in Perl, the double arrow does not treat what comes before it as a bare word, but rather evaluates it. Hence, constants used with the double arrow will be evaluated:
Ruby
In Ruby, the fat comma is the token to create hashes. Ruby 1.9 introduced a special syntax to use symbols as barewords. In Ruby, the fat comma is called a hash rocket.