define-a-custom-rspec-matcher.md

Define A Custom RSpec Matcher

You can define your own custom RSpec matchers. This is a great way to keep your spec tight and readable. A custom, domain-specific matcher can convey more intent in a single line than several lines of built-in matchers.

Here is a matcher to check if something is in a range:

require 'rspec/expectations'

RSpec::Matchers.define :be_betwen do |lower, upper|
  match do |operand|
    expect(operand).to be >= lower
    expect(operand).to be <= upper
  end
end

describe MyThing do
  it "has a value between 0 and 10" do
    thing = MyThing.new

    expect(thing.value).to be_between(0, 10)
  end
end

By requiring rspec/expectations we are able to define a matcher that takes 0 or more arguments (in our case, lower and upper) and then make assertions with them and the expect value.

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