ts-code-contracts

Design by contract with TypeScript.

Installation & Usage

npm i ts-code-contracts

Requires TypeScript^3.7

You can now import the following functions from 'ts-code-contracts':

• requires for preconditions
  • requiresNonNullish for null-checks as preconditions
• checks for invariants
  • checksNonNullish for null-checks as invariants
• ensures for postconditions
  • ensuresNonNullish for null-checks as postconditions
• asserts for impossible events
• unreachable for unreachable code branches
• error to make code more concise
• isDefined type guard

Make sure to checkout the examples in the documentation below or refer to the test cases and typing assistance!

Contracts are really just handy shorthands to throw an error, if the given condition is not met. And yet they greatly help the compiler and the readability of your code.

requires

Use it to validate preconditions, like validating arguments. Throws a PreconditionError if the condition is false.

function requires(
  condition: boolean,
  message: string = 'Unmet precondition'
): asserts condition;

• condition - the condition that should be true
• message - an optional message for the error

Example:

function myFun(name: string) {
  requires(name.length > 10, 'Name must be longer than 10 chars');
}

requiresNonNullish

A variation of requires that returns the given value unchanged if it is not null or undefined. Throws a PreconditionError otherwise.

function requiresNonNullish<T>(
  value: T,
  message = 'Value must not be null or undefined'
): NonNullable<T>;

• value - the value that should not be null or undefined
• message - an optional message for the error

Example:

function myFun(name: string | null) {
  const nameNonNull = requiresNonNullish(name, 'Name must be defined');
  nameNonNull.toUpperCase(); // no compiler error!
}

checks

Use it to check for an illegal state. Throws a IllegalStateError if the condition is false.

function checks(
  condition: boolean,
  message = 'Callee invariant violation'
): asserts condition;

• condition - the condition that should be true
• message - an optional message for the error

Example:

class Socket {
  private isOpen = false;
  send(data: Data) {
    check(this.isOpen, 'Socket must be open');
  }
  open() {
    this.isOpen = true;
  }
}

checksNonNullish

A variation of checks that returns the given value unchanged if it is not null or undefined. Throws a IllegalStateError otherwise.

function checksNonNullish<T>(
  value: T,
  message = 'Value must not be null or undefined'
): NonNullable<T>;

• value - the value that should not be null or undefined
• message - an optional message for the error

Example:

class Socket {
  data: Data | null = null;
  send() {
    const validData = checksNonNullish(this.data, 'Data must be available');
    validData.send(); // no compiler error!
  }
}

ensures

Use it to verify that your code behaved correctly. Throws a PostconditionError if the condition is false.

function ensures(
  condition: boolean,
  message = 'Unmet postcondition'
): asserts condition;

• condition - the condition that should be true
• message - an optional message for the error

Example:

function myFun() {
  createPerson({ id: 0, name: 'John' });
  const entity = findById(0); // returns null if not present
  return ensures(isDefined(entity), 'Failed to persist entity');
}

ensuresNonNullish

A variation of ensures that returns the given value unchanged if it is not null or undefined. Throws a PostconditionError otherwise.

function ensuresNonNullish<T>(
  value: T,
  message = 'Value must not be null or undefined'
): NonNullable<T>;

• value - the value that should not be null or undefined
• message - an optional message for the error

Example:

function myFun(): Person {
  createPerson({ id: 0, name: 'John' });
  const entity = findById(0); // returns null if not present
  return ensuresNonNullish(entity, 'Failed to persist entity');
}

asserts

Clarify that you think that the given condition is impossible to happen. Throws a AssertionError if the condition is false.

asserts(
  condition: boolean,
  message?: string
): asserts condition;

• condition - the condition that should be true
• message - an optional message for the error

unreachable

Asserts that a code branch is unreachable. If it is, the compiler will throw a type error. If this function is reached at runtime, an error will be thrown.

function unreachable(
  value: never,
  message = 'Reached an unreachable case'
): never;

• value - a value
• message - an optional message for the error

Example:

function myFun(foo: MyEnum): string {
  switch (foo) {
    case MyEnum.A:
      return 'a';
    case MyEnum.B:
      return 'b';
    // no compiler error if MyEnum only has A and B
    default:
      unreachable(foo);
  }
}

error

This function will always throw an error. It helps keeping code easy to read and come in handy when assigning values with a ternary operator or the null-safe operators.

function error(message?: string): never;
function error(
  errorType: new (...args: any[]) => Error,
  message?: string
): never;

• errorType - an error class, defaults to IllegalStateError
• message - an optional message for the error

Example:

function myFun(foo: string | null) {
  const bar = foo ?? error(PreconditionError, 'Argument may not be null');
  const result = bar.length > 0 ? 'OK' : error('Something went wrong!');
}

isDefined

A type guard, to check that a value is not null or undefined. Make sure to use strictNullChecks.

function isDefined<T>(value: T): value is NonNullable<T>;

• value - the value to test

Example:

const x: string | null = 'Hello';
if (isDefined(x)) {
  x.toLowerCase(); // no compiler error!
}

Errors

The following error classes are included:

• PreconditionError → An error thrown, if a precondition for a function or method is not met.
• IllegalStateError → An error thrown, if an object is an illegal state.
• PostconditionError → An error thrown, if a function or method could not fulfil a postcondition.
• AssertionError → An error thrown, if an assertion has failed.