RSpec’s be true and be_true look deceptively similar. In fact, their naming suggests that they might be synonyms for the same assertion. Can the two clauses be used interchangeably to produce the same effect? No!
Both check trueness but from different perspectives. be true checks whether the compared-against value is true: “Does the value literally equal true?” be_true asserts that the test value evaluates to true: “Is the value truthy?” be false and be_false are the respective inverse match clauses.
While be true‘s name literally describes what it asserts, be_true‘s name is arguably ambiguous. To prevent future confusion, be_true will be renamed be_truthy and be_false will become be_falsey in the upcoming RSpec 3.0 release.
be true/be false
Asserts that the tested-against expression literally equals true or false (respectively).
it { expect(true).to be true } # passes
it { expect("abc").to be true } # fails
it { expect(nil).to be true } # fails
it { expect(false).to be false } # passes
it { expect("abc").to be false} # fails
it { expect(nil).to be false} # fails
be_true/be_false (soon to be be_truthy/be_falsey)
Asserts that the tested-against expression evaluates to true or false (respectively).
it { expect(true).to be_true } # passes
it { expect("abc").to be_true } # passes
it { expect(nil).to be_true } # fails
it { expect(false).to be_false } # passes
it { expect("abc").to be_false} # fails
it { expect(nil).to be_false } # passes
Interesting! I never knew be true != be_true. I am glad they are renaming it be_truthy to avoid this confusion.
This is definitely one of those things that has tripped people up, and is the reasoning behind making it more explicit in RSpec 3.0 via be_truthy and be_falsy / be_falsey (both are valid aliases of the same matcher method).
Quick note, your post has a rather important typo, “be_true will be renamed be_truthly” (should be be_truthy)
Thanks for catching that typo. Fixed!
Good point about be_falsy being an alias of be_falsey.