Hash#keys

hash.keys

Returns: Array · Added in v1.8 · Updated March 14, 2026 · Hash Methods

ruby stdlib hash

Hash#keys returns a new array containing all the keys from the hash, in insertion order. This method is useful when you need to iterate over all keys or when you need a list of keys for set operations.

Syntax

hash.keys

Examples

Basic key extraction

user = { name: "Alice", email: "alice@example.com", age: 30 }

user.keys
# => [:name, :email, :age]

Iteration with keys

config = { host: "localhost", port: 3000, ssl: true }

config.keys.each do |key|
  puts "#{key}: #{config[key]}"
end

# Output:
# host: localhost
# port: 3000
# ssl: true

Converting to a set

h1 = { a: 1, b: 2 }
h2 = { b: 3, c: 4 }

# Find common keys
(h1.keys & h2.keys)
# => [:b]

Common Patterns

Check if a key exists

data = { name: "Bob", active: true }

data.keys.include?(:name)
# => true

data.keys.include?(:missing)
# => false

However, using Hash#has_key? or Hash#include? is more efficient for this purpose.

Transform keys

hash = { first_name: "John", last_name: "Doe" }

hash.keys.map(&:to_s)
# => ["first_name", "last_name"]

Sorting keys

scores = { bob: 95, alice: 87, charlie: 92 }

scores.keys.sort
# => [:alice, :bob, :charlie]

Get all values as well

h = { x: 10, y: 20 }

h.keys
# => [:x, :y]

h.each_value.to_a
# => [10, 20]

Edge Cases

Empty hash

{}.keys
# => []

Integer keys

{ 1 => "one", 2 => "two" }.keys
# => [1, 2]

Mixed key types

{ "name" => "Alice", :age => 25, 3 => "three" }.keys
# => ["name", :age, 3]