EC
NameError
RubyERRORNotableName

Undefined name or constant

Quick Answer

Ensure constants are defined and required files are loaded before referencing them.

What this means

Raised when a variable, method, or constant name cannot be resolved. It is the parent of NoMethodError. Typically appears when referencing an undefined constant or an undefined local variable.

Why it happens
  1. 1Referencing an undefined constant or class
  2. 2Typo in a constant or variable name
  3. 3Missing require for a gem or file that defines the constant

Fix

Add the missing require

Add the missing require
require 'json'

data = JSON.parse('{"key":"value"}')

Why this works

require loads the file that defines the constant, making it available in the current scope.

Code examples
Undefined constantruby
puts UndefinedClass.new
# NameError: uninitialized constant UndefinedClass
Typo in constantruby
# Intended: ActiveRecord::RecordNotFound
rescue ActiveRecord::RecordNotFoud  # typo
# NameError: uninitialized constant ActiveRecord::RecordNotFoud
Safe constant lookupruby
if Object.const_defined?(:MyClass)
  MyClass.new
else
  puts 'MyClass is not loaded'
end
Sources
Official documentation ↗

Ruby Core Documentation

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