How to check if a specific key is present in a hash or not?

Hash instance has a key? method:

{a: 1}.key?(:a)
=> true

Be sure to use the symbol key or a string key depending on what you have in your hash:

{'a' => 2}.key?(:a)
=> false

While Hash#has_key? gets the job done, as Matz notes here, it has been deprecated in favour of Hash#key?.

hash.key?(some_key)

Hash's key? method tells you whether a given key is present or not.

session.key?("user")

It is very late but preferably symbols should be used as key:

my_hash = {}
my_hash[:my_key] = 'value'

my_hash.has_key?("my_key")
 => false 
my_hash.has_key?("my_key".to_sym)
 => true 

my_hash2 = {}
my_hash2['my_key'] = 'value'

my_hash2.has_key?("my_key")
 => true 
my_hash2.has_key?("my_key".to_sym)
 => false 

But when creating hash if you pass string as key then it will search for the string in keys.

But when creating hash you pass symbol as key then has_key? will search the keys by using symbol.

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If you are using Rails, you can use Hash#with_indifferent_access to avoid this; both hash[:my_key] and hash["my_key"] will point to the same record