predicate-js

Chainable Predicates for JS

The goal is an assertion library that combines Chai's chainable syntax (i.e. has.lengthOf.lessThan(4)), but adds the condition that predicates should be first class objects allowing compositional reuse:

var lt10 = is.lessThan(10);
var gt6 = is.greaterThan(6);
var between6and10 = gt6.and(lt10);

assertThat(7, between6and10);
assertThat(8, between6and10);

Making predicates first class objects brings additional benefits when dealing with anything async. In Chai, this is hard:

expect(someValueWeDontHaveYet).to.have.length.greaterThan(5);

The difficulty is that the assertion and predicate are inexorably linked in Chai. By making predicates first class object like this:

assertThat(someValueWeDontHaveYet, has.length.greaterThan(5));

You keep a nice chainable syntax, but has.length.greaterThan(5) is an object representing the predicate that can be passed around, evaluated when the async result is returned, or combined with other predicates.