Let and before hooks in Ruby - Step-by-Step Execution

Concept Flow - Let and before hooks

Define let variable

↓

Run before hook

↓

Execute test example

↓

Use let variable (lazy evaluated)

↓

Test completes

This flow shows how a let variable is defined lazily, before hooks run setup code, and then the test example uses the let variable.

Execution Sample

Ruby

let(:number) { 5 }

before do
  @result = number * 2
end

it "doubles the number" do
  expect(@result).to eq(10)
end

This code defines a let variable 'number', runs a before hook to set @result, then tests that @result equals 10.

Execution Table

Step	Action	Evaluation	Result
1	Define let(:number)	No evaluation yet	number is ready but not called
2	Run before hook	Calls number for first time	number evaluated to 5, @result set to 10
3	Run test example	Use @result	@result is 10, test passes
4	Test ends	No more code	Test complete

💡 Test ends after example runs and assertions pass

Variable Tracker

Variable	Start	After before hook	After test example	Final
number (let)	not evaluated	evaluated to 5	evaluated to 5	evaluated to 5
@result	nil	10	10	10

Key Moments - 3 Insights

Why is the let variable 'number' not evaluated when defined?

Because let variables are lazy; they only run when first called, as shown in step 2 of the execution_table.

When does the before hook run in relation to the test example?

The before hook runs before the test example, setting up variables like @result, as seen in step 2 before step 3.

Can the let variable be used inside the before hook?

Yes, the let variable is evaluated the first time it is called, even inside the before hook, as step 2 shows.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the let variable 'number' first evaluated?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Check the 'Evaluation' column in execution_table rows for when 'number' is evaluated.

According to variable_tracker, what is the value of @result after the before hook runs?

A 5

B 10

C nil

D undefined

💡 Hint

Look at the '@result' row under 'After before hook' column in variable_tracker.

If the before hook did not call 'number', when would 'number' be evaluated?

A Never

B At test definition time

C When the test example calls it

D Before the before hook

💡 Hint

Recall that let variables are lazy and only evaluated when first used, as shown in the flow and execution_table.

Concept Snapshot

let(:name) { value } defines a lazy variable.
before hooks run setup code before each test.
let variables are evaluated only when first called.
before hooks can use let variables, triggering evaluation.
Test examples use variables set up by before hooks and let.
This helps organize test setup cleanly and efficiently.

Full Transcript

This visual execution trace shows how Ruby's let and before hooks work in tests. First, a let variable 'number' is defined but not evaluated immediately. Then, the before hook runs, calling 'number' for the first time, which evaluates it to 5 and sets @result to 10. Next, the test example runs, using @result which is 10, so the test passes. The variable tracker shows 'number' changes from not evaluated to 5 after the before hook, and @result changes from nil to 10. Key moments clarify that let variables are lazy and evaluated only when called, including inside before hooks. The quizzes check understanding of when evaluation happens and variable values. The snapshot summarizes that let defines lazy variables, before hooks run setup before tests, and together they organize test code cleanly.