Hexagonal architecture, also called Ports and Adapters or Onion architecture, keeps application code behind internal abstractions instead of importing uncontrolled dependencies directly. Replaceable adapters connect those abstractions to libraries and other external systems. section 3.5 section 5.2.2
The practical payoff is less brittle testing and safer replacement. A logging library or date library can change behind one adapter while application code and tests continue using a smaller API that the team controls. Direct mocking can solve legacy code problems, but it also binds tests to third-party APIs. section 3.5 section 5.2.2
Wrapping a date dependency
Instead of letting verifyPassword import moment directly, place moment behind a controlled dependency boundary. Tests can inject a small fake with a day function, verify the weekend behavior, and reset the dependency afterward. The test checks the internal contract rather than reproducing moment.js's entire API. section 3.5 section 3.5
Listing 3.7 Abstracting the required dependencies
const originalDependencies = { #1
moment: require(‘moment’), #1
}; #1
let dependencies = { ...originalDependencies }; #2
const inject = (fakes) => { #3
Object.assign(dependencies, fakes);
return function reset() { #4
dependencies = { ...originalDependencies };
}
};
const SUNDAY = 0; const SATURDAY = 6;
const verifyPassword = (input, rules) => {
const dayOfWeek = dependencies.moment().day();
if ([SATURDAY, SUNDAY].includes(dayOfWeek)) {
throw Error("It's the weekend!");
}
// more code goes here...
// return list of errors found..
return [];
};
module.exports = {
SATURDAY,
verifyPassword,
inject
};
#1 Wrapping moment.js with an intermediary object
#2 The object containing the current dependency, either real or fake
#3 A function that replaces the real dependency with a fake one
#4 A function that resets the dependency back to the real one
A compact JavaScript listing shows a wrapped dependency, an injectable fake, and a reset function for restoring the original implementation. section 3.5
