RSpec Let vs Before

In RSpec, there are two different ways to write DRY tests, by using before or let. Their purpose is to create variables that are common across tests. In this post, we will explore differences between before and let and explain why let is preferred by the Ruby community.

let

let creates lazily-evaluated local variables. This means that let() is not evaluated until the method that it formed is run for the first time. It DRYs up the spec and makes it more readable.

$count = 0
describe "let" do
  let(:count) { $count += 1 }

  it "stores the value" do
    expect(count).to eq(1)
    expect(count).to eq(1)
  end

  it "is not cached across examples" do
    expect(count).to eq(2)
  end
end

let should not be used for local variables, which have to be saved to the database, as they will not be saved to the database unless they have already been referenced. In this case, you should use let! or before blocks.

Also, never have a let block inside of a before the block, this is what let! is made for!

let!

Unlike let, you can use let! to force the method's invocation before each example. It means that even if you didn't invoke the helper method inside the example it will be invoked before your example runs.

$count = 0
describe "let!" do
  invocation_order = []

  let!(:count) do
    invocation_order << :let!
    $count += 1
  end

  it "calls the helper method in a before hook" do
    invocation_order << :example
    expect(invocation_order).to eq([:let!, :example])
    expect(count).to eq(1)
  end
end

As with let blocks, if multiple let! blocks are defined with the same name, the most recent one will be executed. The core difference is that  let! blocks will be executed multiple times if used like this, whereas the let block will only execute the last time.

before (: each)

The before (: each) block will run before each example, even if the example does not use any of the instance variables defined in the block. This can noticeably slow down the setup of the instance variables.

class User
  def tests
    @tests ||= []
  end
end

describe User do
  before(:each) do
    @user = User.new
  end

  describe "initialized in before(:each)" do
    it "has 0 tests" do
      @user.should have(0).tests
    end

    it "can accept new tests" do
      @user.tests << Object.new
    end

    it "does not share state across examples" do
      @user.should have(0).tests
    end
  end
end

In nearly every situation, it is better to use let over before blocks. Depending on your personal preference you could use before blocks when:

• There is a reasonable amount of variables.
• There are variables that don't need to be referenced directly but are required.
• There are many commands to be executed because its syntax is clearer when many commands are involved.
• Creating mocks/stubs.

before (: all)

This block is executed only once, before all of the examples in a group. There are certain situations this can cut down on execution and effort.

class User
  def tests
    @tests ||= []
  end
end

describe User do
  before(:all) do
    @user = User.new
  end

  describe "initialized in before(:all)" do
    it "has 0 tests" do
      @user.should have(0).tests
    end

    it "can get accept new tests" do
      @user.tests << Object.new
    end

    it "shares state across examples" do
      @user.should have(1).tests
    end
  end
end

Using runs (: all) in RSpec will cause you lots of trouble unless you know what you are doing! It runs outside of transactions, so the data created here will bleed into other specs.

Conclusion

Let blocks bring more to the table than before blocks. It all depends on what and how you need to make the RSpec tests.

Besides being slower, one of the major problems with before blocks is that spelling errors can lead to bugs and false positives, allowing certain types of tests to pass when they shouldn't.

before(:each) do
    @user = User.find(username: "kolosek")
    @user.logout
end

it "should log the user out" do
    expect(@usr).to be_nil
end

Since @usr was not previously defined, the test will pass, @usr is nil by default. The same test using let would raise NameError because @usr is not defined.

Hope this will help you to better understand the differences between let and before blocks.

Thank you for reading!