Convert Strings Between Snake_case And Camelcase In Ruby
When bridging Ruby code (which uses snake_case) with JavaScript APIs or JSON payloads (which often use camelCase), you regularly need to convert between the two formats. Ruby and Rails provide clean ways to do both directions.
Description
Rails ActiveSupport adds camelize and underscore to String for converting between snake_case and CamelCase. In plain Ruby without ActiveSupport, a few targeted gsub calls handle both directions cleanly.
camelize by default produces UpperCamelCase (PascalCase). Pass false or :lower to get lowerCamelCase for JSON keys.
These conversions are essential when serializing Ruby hashes to JSON for JavaScript frontends, or when parsing incoming camelCase params from a JavaScript client into Rails-friendly snake_case keys.
Sample input:
"user_first_name" # snake → camel
"userFirstName" # camel → snake
Sample Output:
"UserFirstName" # UpperCamelCase
"userFirstName" # lowerCamelCase
"user_first_name" # snake_case
Answer
# With ActiveSupport (Rails)
"user_first_name".camelize # => "UserFirstName"
"user_first_name".camelize(:lower) # => "userFirstName"
"userFirstName".underscore # => "user_first_name"
# Plain Ruby — no ActiveSupport
def to_camel_case(str, upper: true)
result = str.gsub(/_([a-z])/) { $1.upcase }
upper ? result.sub(/^(.)/) { $1.upcase } : result
end
def to_snake_case(str)
str.gsub(/([A-Z]+)([A-Z][a-z])/, '\1_\2')
.gsub(/([a-z\d])([A-Z])/, '\1_\2')
.downcase
end
to_camel_case("user_first_name") # => "UserFirstName"
to_camel_case("user_first_name", upper: false) # => "userFirstName"
to_snake_case("UserFirstName") # => "user_first_name"
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