Ruby-on-rails – Capybara and Rspec: correct way to use within() and have_selector() together

I always prefer let to an instance variable for a couple of reasons:

• Instance variables spring into existence when referenced. This means that if you fat finger the spelling of the instance variable, a new one will be created and initialized to nil, which can lead to subtle bugs and false positives. Since let creates a method, you'll get a NameError when you misspell it, which I find preferable. It makes it easier to refactor specs, too.
• A before(:each) hook will run before each example, even if the example doesn't use any of the instance variables defined in the hook. This isn't usually a big deal, but if the setup of the instance variable takes a long time, then you're wasting cycles. For the method defined by let, the initialization code only runs if the example calls it.
• You can refactor from a local variable in an example directly into a let without changing the referencing syntax in the example. If you refactor to an instance variable, you have to change how you reference the object in the example (e.g. add an @).
• This is a bit subjective, but as Mike Lewis pointed out, I think it makes the spec easier to read. I like the organization of defining all my dependent objects with let and keeping my it block nice and short.

A related link can be found here: http://www.betterspecs.org/#let

I understood the difference between let and let! with a very simple example. Let me read the doc sentence first, then show the output hands on.

About let doc says :-

... let is lazy-evaluated: it is not evaluated until the first time the method it defines is invoked.

I understood the difference with the below example :-

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

  it "returns 1" do
    expect($count).to eq(1)
  end
end

Lets run it now :-

arup@linux-wzza:~/Ruby> rspec spec/test_spec.rb
F

Failures:

  1) let is not cached across examples
     Failure/Error: expect($count).to eq(1)

       expected: 1
            got: 0

       (compared using ==)
     # ./spec/test_spec.rb:8:in `block (2 levels) in <top (required)>'

Finished in 0.00138 seconds (files took 0.13618 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./spec/test_spec.rb:7 # let is not cached across examples
arup@linux-wzza:~/Ruby>

Why the ERROR ? Because, as doc said, with let, it is not evaluated until the first time the method it defines is invoked. In the example, we didn't invoke the count, thus $count is still 0, not incremented by 1.

Now coming to the part let!. The doc is saying

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

Lets test this also :-

Here is the modified code

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

  it "returns 1" do
    expect($count).to eq(1)
  end
end

Lets run this code :-

arup@linux-wzza:~/Ruby> rspec spec/test_spec.rb
.

Finished in 0.00145 seconds (files took 0.13458 seconds to load)
1 example, 0 failures

See, now $count returns 1, thus test got passed. It happened as I used let!, which run before the example run, although we didn't invoke count inside our example.

This is how let and let! differs from each other.