ts-design-by-contract

Description

This package contains a set of three decoratos: PreCon , PostCon, Invariant

Installation

Follow these steps to install the package:

Prerequisites

• Node.js (>= version 22.0.0)
• npm (>= version 10.5.0)

Installation with npm

npm i ts-design-by-contract

Decorator PreCon

Description: The Precondition decorator is used to enforce preconditions on method inputs in TypeScript classes. By specifying a condition function that evaluates the method's arguments, you can ensure that certain criteria are met before the method execution. If the condition fails, an error is thrown with a provided message. This decorator aligns with the Design by Contract principles, ensuring the integrity of the method's preconditions.

Usage:

@PreCon((value: number) => value > 0, "Value must be greater than 0")
method(value: number): number {
  return value + 1;
}

Decorator PostCon

Description: The PostCon decorator is used to enforce postconditions on method outputs in TypeScript classes. By specifying a condition function that evaluates the result of a method, you can ensure that certain criteria are met after the method execution. If the condition fails, an error is thrown with a provided message. This decorator follows the Design by Contract principles, ensuring the integrity of the method's postconditions.

Usage:

@PostCon((result: number) => result > 0, "Result must be greater than 0")
method(value: number): number {
  return value - 1;
}

Decorator Invariant

Description: The Invariant decorator is used to enforce class invariants in TypeScript classes. It checks the invariants both before and after a method execution by invoking a checkInvariants method on the class instance. If the invariants are violated, an error is thrown. This decorator helps maintain the integrity of the class state according to the Design by Contract principles, ensuring that invariants hold throughout the object's lifetime.

Usage:

// Don't forget to implement interface ICheckInvariant
class Example implements ICheckInvariant {
  private _value: number;

  constructor(value: number) {
    this._value = value;
    this.checkInvariants();
  }

  @Invariant
  setValue(value: number) {
    this._value = value;
  }

  @Invariant
  incrementValue() {
    this._value++;
  }

  checkInvariants() {
    if (this._value < 0) {
      throw new Error("Invariant violation: _value must be non-negative");
    }
  }
}