Invariant and non-null/undefined assertions, both

• strict, i.e. logs error and throws exception, and
• soft, i.e. just logs a warning.

Both strict and soft will narrow type, i.e. eliminate null or undefined.

The purpose of this library is to make assumptions explicit, rather than just a comment or even worse just a thought while writing your code. This applies both to assumptions about conditions to be met or values not being null/undefined.

To install the library into your project, run yarn or npm:

yarn add assert-ts

or

npm i assert-ts

import assert from 'assert-ts';

function transfer(fromId: string, toId: string, amount: number) {
  // Throws error if not true
  assert(amount > 0);
  ...
}

import assert from 'assert-ts';

function transfer(fromId: string, toId: string, amount: number) {
  // Logs warning if false
  if (assert.soft(amount > 0)) {
    ...
  }
}

To make it easier to find the cause of an assertion failure, you can provide more information, i.e. a custom message and any relevant properties.

import assert from 'assert-ts';

function transfer(fromId: string, toId: string, amount: number) {
  // Custom message and properties will be formatted into error message
  assert(amount > 0, "Cannot transfer 0 or negative amounts", { fromId, toId, ammount });
  ...
}

import assert from 'assert-ts';

function findAccount(id): Account | undefined { ... }

function transfer(fromnId: string, toId: string, amount: number) {
  ...

  // Throws error if findAccount returns undefined
  const fromAccount = assert(findAccount(fromId), "From account does not exist", { fromId});

  // Type restriction: when a non-null/undefined assertion succeeds,
  // type is restricted, e.g. to Account. Hence, no need for further testing of undefined/null
  fromAccount.amount -= amount;

  ...
}

import assert from 'assert-ts';


function notify(person?: Person) {
  ...
  // Logs warning if person is undefined
  if (assert.soft(person, 'Person should be provided')) {
    // person narrowed to Person
    sendSms(person.mobile);
  }
}

assert has two signatures

Checks that a condition is true. If not, an error is thrown. By default, any message or properties provided will be formatted as part of the error's message. See below for custom configuration.

function assert(
  condition: boolean,
  message?: string,
  props?: object | (() => object),
): asserts condition;

Checks that a value is not null or undefined. If null or undefined, an error is thrown. When successful, the returned value's type is restricted to the expected type.

function assert<T>(
  value: T | undefined | null,
  message?: string,
  props?: object | (() => object),
): T;

assert.soft has two signatures

Checks that a condition is true. If not, logs a warning. By default, any message or properties provided will be formatted as part of the warning message. See below for custom configuration.

function soft(
  condition: boolean,
  message?: string,
  props?: object | (() => object),
): condition is true;

Checks that a value is not null or undefined. If null or undefined, a warning is logged. When successful, the value's type is narrowed to the expected type.

function soft<T>(
  value: T | undefined | null,
  message?: string,
  props?: object | (() => object),
): value is T;

The default configuration throws an Error with a message saying whether it was a condition or null/undefined check that failed and any custom message or properties formatted as part of the message.

Use configureAssert to customize this, providing an AssertConfiguration object with any of the following properties:

Bjørn Egil Hansen (@bjornegil)

Fram X - a cross platform app company from Norway.