Rspec is a great ruby library for writing tests (or “specs” as they are called in Rspec). It includes lots of great helper methods that can help you save time and write faster more performant tests.

Often times when you’re writing a test you need to prepare some data or do some action before that test gets run.

RSpec provides 2 main ways of achieving this:

1. Via the let statement.
2. Via the before :each statement.

But what’s the difference between the two?

In short, let blocks are “lazy” and only gets evaluated the first time they are called in your tests, and before :each blocks are run before every test (within the same context/describe block it’s written in), regardless of whether you use or need what’s done inside them.

Let’s see a very simple example in order to understand the difference.

In the following code, we create two user instances and then we do a test on one of them. Let’s see the difference between let and before :each in this case.

Example using let:

let(:user_without_name) { User.new(name: nil, age: 28, gender: "Male" ) }
let(:user_without_age) { User.new(name: "Paz", age: nil, gender: "Male" ) }

it "fails to save the user if the name is blank" do
  user_without_name.save
  expect(user_without_name.errors.keys).to include(:name)
end

Example using before :each:

before :each do
  @user_without_name = User.new(name: nil, age: 28, gender: "Male" )
  @user_without_age = User.new(name: "Paz", age: nil, gender: "Male" )
end

it "fails to save the user if the name is blank" do
  @user_without_name.save
  expect(@user_without_name.errors.keys).to include(:name)
end

In the let example the “user_without_name” is the only instance created when running the test as the “user_without_age” never gets called in the test.

However, In the before :each example both the “@user_without_name” and the “@user_without_age” are created when the test is run even though “@user_without_age” never gets called.

When should you use each one?

When you need to decide if you should use let or before :each in your test, think if you will need everything that runs inside your block to happen before every test. If the answer is no - use let.