@xility/invariant

A simple assertion utility function.

Installation

npm install @xility/invariant

Example

Simple argument validation

import { invariant } from "@xility/invariant;

function greet(name) {
  invariant(name, "name is required");
  return `Hello, ${name}!`;
}

greet("Bob"); // returns "Hello, Bob!"
greet(null); // throws Error("name is required")

Type narrowing/assertion

const person: Person | undefined = await db.getPerson(id);

invariant(person, "person is required");

// the type system knows that person is NOT undefined here
return `Hello, ${person.name}!`;

Custom Error type

class ValidationError extends Error {
  constructor(message: string) {
    super(message);
  }
}

invariant(amount > 0, "amount must be greater than zero", ValidationError);