EC
NoMethodError
RubyFATALCommonMethod

Undefined method called on object

Quick Answer

Check the object type with .class and use respond_to? or the safe navigation operator &. to guard against nil.

What this means

Raised when a method is called on an object that does not respond to it. This is one of the most common Ruby errors, frequently caused by calling a method on nil or a type mismatch.

Why it happens
  1. 1Calling a method on nil when a variable was expected to hold an object
  2. 2Typo in method name
  3. 3Method defined in a module that was not included
  4. 4Calling a private method from outside the class

Fix 1

Safe navigation operator

Safe navigation operator
user = find_user(id)   # may return nil
name = user&.name       # returns nil safely instead of raising

Why this works

The &. operator short-circuits and returns nil if the receiver is nil, preventing NoMethodError.

Fix 2

Guard with respond_to?

Guard with respond_to?
if obj.respond_to?(:process)
  obj.process
else
  raise TypeError, "Expected a processable object, got #{obj.class}"
end

Why this works

respond_to? checks at runtime whether the method exists before calling it.

Code examples
Reproducing the errorruby
nil.upcase
# NoMethodError: undefined method 'upcase' for nil
Safe navigation operatorruby
user = nil
puts user&.name   # => nil, no error raised
Rescue NoMethodErrorruby
begin
  result = obj.missing_method
rescue NoMethodError => e
  puts "Error: #{e.message}"
end
Sources
Official documentation ↗

Ruby Core Documentation

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