Kernel#block_given?

block_given? -> true or false

Returns: true or false · Updated March 13, 2026 · Core Classes

blocks control-flow kernel

block_given? (also available as iterator?) is a Kernel method that checks whether the current method was called with a block. It returns true if a block is associated with the current call, and false otherwise. This method is essential for writing flexible methods that can behave differently depending on whether a caller provides a block.

Syntax

block_given?

Parameters

This method takes no parameters.

Examples

Basic usage

def greet
  if block_given?
    yield
  else
    puts "Hello!"
  end
end

greet { puts "Hello from the block!" }
# Hello from the block!

greet
# Hello!

Checking for a block before yielding

def process_items(items)
  if block_given?
    items.map { |item| yield item }
  else
    items
  end
end

result = process_items([1, 2, 3]) { |n| n * 2 }
# => [2, 4, 6]

result = process_items([1, 2, 3])
# => [1, 2, 3]

Common Patterns

Optional block with custom behavior

def configure
  config = { timeout: 30, retries: 3 }
  
  if block_given?
    yield(config)
  end
  
  config
end

config = configure do |c|
  c[:timeout] = 60
  c[:retries] = 5
end
# => { timeout: 60, retries: 3 }

config = configure
# => { timeout: 30, retries: 3 }

Building strings conditionally

def build_message(base)
  message = base
  
  if block_given?
    message = yield(message)
  end
  
  message
end

build_message("Hello") { |m| m + ", World!" }
# => "Hello, World!"

build_message("Hello")
# => "Hello"

Errors

Calling yield without a block raises a LocalJumpError. Always check block_given? before using yield to prevent this error.

def unsafe
  yield  # Raises LocalJumpError if no block given
end

def safe
  yield if block_given?  # Safe
end