Hashes and symbols

A Hash is Ruby's dictionary: it maps each key to a value, like user = { name: "Layan", city: "Riyadh" }. The keys here are symbols (Symbol), and name: is shorthand for :name =>. A symbol is a fixed name, and every :name in a program is the very same object, which is why Ruby usually uses symbols as keys. But a symbol isn't equal to a string, so user["name"] returns nil, because the key is :name.

If you ask for a key that doesn't exist, you won't get an error: user[:plan] quietly returns nil. fetch(:plan, :basic), on the other hand, returns the value if the key exists and the default if it doesn't, and key?(:plan) asks explicitly whether the key is there. Watch out for user[:newsletter] || true: if the stored value is false, || replaces it with true and overrides the user's choice, while fetch tells a missing key apart from a key whose value is false.

You loop over the key and value pairs with each { |key, value| ... }. Note that map on a hash returns an array, not a hash, while transform_values returns a new hash with the same keys and changed values.