Conveying Intent with RSpec subject

Today during a test refactor, I discovered RSpec's subject method. We used it a lot, because it helps convey the test's intent.

subject can be used as a convenience method, and to define the intent of a test. If you've ever seen a comment # This is what we are testing, subject would be an upgrade to that.

Here are three passing examples of this method in practice, from most to least verbose.

Explicit, with a named subject:

RSpec.describe Array, "with some elements, explicit named subject" do
  subject(:array) { [1, 2, 3] }
  it "should have the prescribed elements" do
    expect(array).to eq [1, 2, 3]
  end
end

Explicit, with no name:

RSpec.describe Array, "with some elements, explicit subject" do
  subject { [1, 2, 3] }
  it "should have the prescribed elements" do
    expect(subject).to eq [1, 2, 3]
  end
end

Implicit:

RSpec.describe Array, "with some elements, implicit subject" do
  it "should have the default elements" do
    expect(subject).to be_empty
  end
end

Substitute this for let on your described class. There are a ton of interesting uses and edges for this method, so dig into those docs.

h/t Mike Chau

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