A fluent API for enforcing design contracts with automatic message generation:

✔️ Easy to use
✔️ Fast
✔️ Production-ready

To get started, add this dependency:

npm install --save @cowwoc/requirements@4.0.8

or pnpm:

pnpm add @cowwoc/requirements@4.0.8

import {requireThatString} from "@cowwoc/requirements";

class Cake
{
	private bitesTaken = 0;
	private piecesLeft;

	public constructor(piecesLeft: number)
	{
		requireThat(piecesLeft, "piecesLeft").isPositive();
		this.piecesLeft = piecesLeft;
	}

	public eat(): number
	{
		++bitesTaken;
		assertThat(bitesTaken, "bitesTaken").isNotNegative().elseThrow();

		piecesLeft -= ThreadLocalRandom.current().nextInt(5);

		assertThat(piecesLeft, "piecesLeft").isNotNegative().elseThrow();
		return piecesLeft;
	}

	public getFailures(): String[]
	{
		return checkIf(bitesTaken, "bitesTaken").isNotNegative().
			and(checkIf(piecesLeft, "piecesLeft").isGreaterThan(3)).
			elseGetMessages();
	}
}

If you violate a precondition:

const cake = new Cake(-1000);

You'll get:

RangeError: "piecesLeft" must be positive.
actual: -1000

If you violate a class invariant:

const cake = new Cake(1_000_000);
while (true)
	cake.eat();

You'll get:

lang.AssertionError: "bitesTaken" may not be negative.
actual: -128

If you violate a postcondition:

const cake = new Cake(100);
while (true)
	cake.eat();

You'll get:

AssertionError: "piecesLeft" may not be negative.
actual: -4

If you violate multiple conditions at once:

const cake = new Cake(1);
cake.bitesTaken = -1;
cake.piecesLeft = 2;
const failures = [];
for (const failure of cake.getFailures())
	failures.add(failure);
console.log(failures.join("\n\n"));

You'll get:

"bitesTaken" may not be negative.
actual: -1

"piecesLeft" must be greater than 3.
actual: 2

This library offers the following features:

• Automatic message generation for validation failures
• Diffs provided whenever possible to highlight the differences between expected and actual values
• Zero overhead when assertions are disabled for better performance
• Multiple validation failures that report all the errors at once
• Nested validations that allow you to validate complex objects
• String diff that shows the differences between two strings

Designed for discovery using your favorite IDE's auto-complete feature. The main entry points are:

• requireThat(value, name) for method preconditions.
  • requireThat(value, name)
  • requireThatArray(value, name)
  • requireThatBoolean(value, name)
  • requireThatMap(value, name)
  • requireThatNumber(value, name)
  • requireThatSet(value, name)
  • requireThatString(value, name)
• assertThat(value, name) for class invariants, method postconditions and private methods.
  • assertThat(value, name)
  • assertThatArray(value, name)
  • assertThatBoolean(value, name)
  • assertThatMap(value, name)
  • assertThatNumber(value, name)
  • assertThatSet(value, name)
  • assertThatString(value, name)
• checkIf(value, name) for multiple failures and customized error handling.
  • checkIf(value, name)
  • checkIfArray(value, name)
  • checkIfBoolean(value, name)
  • checkIfMap(value, name)
  • checkIfNumber(value, name)
  • checkIfSet(value, name)
  • checkIfString(value, name)

See the API documentation for more details.

• Use checkIf().elseGetMessages() to return failure messages without throwing an exception. This is the fastest validation approach, ideal for web services.
• To enhance the clarity of failure messages, you should provide parameter names, even when they are optional. In other words, favor assert that(value, name) over assert that(value).

• http://chaijs.com/
• https://github.com/dsheiko/bycontract
• https://github.com/muroc/offensive.js

Code licensed under the Apache License, Version 2.0: http://www.apache.org/licenses/LICENSE-2.0

Icons made by Flat Icons from www.flaticon.com is licensed by CC 3.0 BY