The Key Differences Between let and let! in RSpec

In RSpec, let and let! define memoized helper methods. They are both helpful in setting up test values but differ fundamentally.

let

• Lazy Evaluation: let is lazily evaluated, meaning the variable is only created when it is first called within an example.
• Reusability: It caches the value within the context of the test example, so multiple calls to the same let within an example will return the same object.
• Scope: Useful in defining unnecessary variables for every test.

Example:

let(:user) { User.new(name: 'John Doe') }

it 'creates a new user' do
  expect(user.name).to eq('John Doe')
end

In this example, the user object is only created when user is first called.

let!

• Eager Evaluation: let! runs the block before each example, ensuring the variable is instantiated regardless of whether it's used in the test.
• Purpose: This is ideal for setup code that you need to run for every test but don’t explicitly call.

Example:

let!(:user) { User.create(name: 'Jane Doe') }

it 'creates a user in the database' do
  expect(User.count).to eq(1)
end

Here, the user object is created before each example runs.

Overriding Variables in Nested Contexts

let and let! can be redefined in nested contexts (e.g., within describe or context blocks). However, this does not "override" the original variable but creates a new instance specific to that nested context.

Example:

describe 'user creation' do
  let(:user) { User.new(name: 'John Doe') }

  context 'when overriding user in a nested context' do
    let(:user) { User.new(name: 'Jane Doe') }

    it 'uses the overridden user' do
      expect(user.name).to eq('Jane Doe')
    end
  end

  it 'uses the original user' do
    expect(user.name).to eq('John Doe')
  end
end

In this case, the user variable is defined twice. The nested context block appears to "override" the outer variable, but this change is limited to the inner block. The outer context remains unaffected.

Summary

• Use let when you want to define a variable lazily, and it may not be needed in every test.
• Use let! for code that needs to run before each example, regardless of whether the variable is used.
• You can redefine variables in nested contexts with let or let!, but this does not affect the outer context.