RSpec expectations and matchers in Ruby

Introduction

RSpec expectations let you check if your code works as you want. Matchers help describe what you expect.

When you want to test if a method returns the right value.

When you want to check if an object has a certain property.

When you want to verify if a block of code raises an error.

When you want to confirm that a value is included in a list.

When you want to test if two values are equal or not.

Syntax

Ruby

expect(actual_value).to matcher(expected_value)
expect(actual_value).not_to matcher(expected_value)

expect() wraps the actual value you want to test.

to means you expect the matcher to be true; not_to means you expect it to be false.

Examples

Checks if 5 equals 5.

Ruby

expect(5).to eq(5)

Checks if the string 'hello' contains 'ell'.

Ruby

expect('hello').to include('ell')

Checks that the array does not include 4.

Ruby

expect([1, 2, 3]).not_to include(4)

Checks if the block raises a StandardError.

Ruby

expect { raise StandardError }.to raise_error(StandardError)

Sample Program

This program tests simple math operations using RSpec expectations and matchers.

It checks addition, array inclusion, and error raising.

Ruby

require 'rspec/autorun'

RSpec.describe 'Simple math' do
  it 'adds numbers correctly' do
    expect(2 + 3).to eq(5)
  end

  it 'does not equal wrong sum' do
    expect(2 + 2).not_to eq(5)
  end

  it 'includes element in array' do
    expect([1, 2, 3]).to include(2)
  end

  it 'raises error when dividing by zero' do
    expect { 1 / 0 }.to raise_error(ZeroDivisionError)
  end
end

Important Notes

Matchers like eq, include, and raise_error are common and easy to use.

You can chain not_to to expect something NOT to happen.

RSpec output shows which tests passed or failed, helping you find problems quickly.

Summary

RSpec expectations check if your code behaves as expected.

Matchers describe the exact condition you want to test.

Use expect().to for positive checks and expect().not_to for negative checks.

Practice

1. What does the RSpec matcher expect(value).to eq(5) check for?

easy

A. It checks if value is less than 5.

B. It checks if value is greater than 5.

C. It checks if value is not equal to 5.

D. It checks if value is equal to 5.

Solution

1. Step 1: Understand the eq matcher

   The eq matcher tests for equality between the expected and actual values.

2. Step 2: Analyze the expectation

   expect(value).to eq(5) means the test expects value to be exactly 5.

3. Final Answer:

   It checks if value is equal to 5. -> Option D

4. Quick Check:

   eq matcher = equality check [OK]

Hint: Remember eq means exact equality check [OK]

Common Mistakes:

• Confusing eq with greater_than or less_than
• Thinking eq checks for inequality
• Mixing up expect().to and expect().not_to

2. Which of the following is the correct syntax to check that an array includes the number 3 in RSpec?

easy

A. expect(array).to has(3)

B. expect(array).to include(3)

C. expect(array).to contains(3)

D. expect(array).to has_key(3)

Solution

1. Step 1: Identify the correct matcher for inclusion

   The include matcher checks if a collection contains a specific element.

2. Step 2: Match syntax with RSpec standards

   expect(array).to include(3) is the correct syntax to check if 3 is in the array.

3. Final Answer:

   expect(array).to include(3) -> Option B

4. Quick Check:

   include matcher = check element presence [OK]

Hint: Use include to check if collection has an element [OK]

Common Mistakes:

• Using has or contains which are not valid RSpec matchers
• Using has_key which is for hashes, not arrays
• Forgetting to use expect().to syntax

3. What will be the output of this RSpec test snippet?

describe 'Number test' do
  it 'checks if number is odd' do
    number = 7
    expect(number.odd?).to be true
  end
end

medium

A. The test passes because 7 is odd.

B. The test fails because 7 is not odd.

C. SyntaxError due to incorrect matcher usage.

D. The test passes but warns about deprecated syntax.

Solution

1. Step 1: Understand the code logic

   The variable number is set to 7, which is an odd number.

2. Step 2: Analyze the expectation

   expect(number.odd?).to be true checks if number.odd? returns true, which it does for 7.

3. Final Answer:

   The test passes because 7 is odd. -> Option A

4. Quick Check:

   odd? returns true for odd numbers [OK]

Hint: odd? returns true for odd numbers, expect that [OK]

Common Mistakes:

• Assuming 7 is even
• Confusing be true with eq true
• Thinking syntax is wrong here

4. Identify the error in this RSpec expectation:

expect(user.name).to eq nil

medium

A. Should use not_to eq nil to check for nil.

B. Missing parentheses around eq(nil).

C. Should use be_nil matcher instead of eq nil.

D. No error; the syntax is correct.

Solution

1. Step 1: Understand nil checking in RSpec

   While eq nil works, the preferred matcher for nil is be_nil for clarity and style.

2. Step 2: Identify best practice

   Using expect(user.name).to be_nil is clearer and idiomatic in RSpec.

3. Final Answer:

   Should use be_nil matcher instead of eq nil. -> Option C

4. Quick Check:

   Use be_nil for nil checks [OK]

Hint: Use be_nil matcher for nil checks, not eq nil [OK]

Common Mistakes:

• Thinking eq nil is a syntax error
• Adding unnecessary parentheses
• Using not_to eq nil to check for nil

5. You want to test that a method calculate returns a value greater than 10 but less than 20. Which RSpec expectation correctly checks this?

hard

A. expect(calculate).to satisfy { |v| v > 10 && v < 20 }

B. expect(calculate).to be > 10 and be < 20

C. expect(calculate).to be_between(10, 20).exclusive

D. expect(calculate).to be_between(11, 19).inclusive

Solution

1. Step 1: Understand the range requirement

   The value must be greater than 10 and less than 20, excluding 10 and 20 themselves.

2. Step 2: Analyze each option

   expect(calculate).to be > 10 and be < 20: expect(calculate).to be > 10 and be < 20 is invalid syntax; correct chaining uses (be > 10).and(be < 20).
   Options C and D: be_between is not a built-in RSpec matcher.
   expect(calculate).to satisfy { |v| v > 10 && v < 20 }: expect(calculate).to satisfy { |v| v > 10 && v < 20 } correctly checks the condition using a standard flexible matcher.

3. Step 3: Choose the best matcher

   expect(calculate).to satisfy { |v| v > 10 && v < 20 } is the correct and explicit choice using standard RSpec.

4. Final Answer:

   expect(calculate).to satisfy { |v| v > 10 && v < 20 } -> Option A

5. Quick Check:

   satisfy matcher = custom condition check [OK]

Hint: Use satisfy with block for complex conditions [OK]

Common Mistakes:

• Using non-standard be_between matcher
• Trying to chain matchers with 'and' incorrectly
• Ignoring boundary conditions in range checks