Type Guardian

Type Guardian is a lightweight, zero-dependency TypeScript library that provides a collection of type guards and assertions to help you write safer and more readable code.

Installation

Install the package using your favorite package manager:

npm install type-guardians
# or
yarn add type-guardians
# or
pnpm add type-guardians

Usage

Type Guardian offers two types of functions for each data type: is functions for type guarding and assert functions for type assertions.

is functions

is functions are type guards that return a boolean value indicating whether the input value is of the expected type. You can use them in if statements to narrow down the type of a variable.

import { isString } from "type-guardians/string";

function greet(name: unknown) {
  if (isString(name)) {
    // name is now of type string
    console.log(`Hello, ${name}!`);
  } else {
    console.log("Hello, guest!");
  }
}

assert functions

assert functions are assertions that throw a TypeError if the input value is not of the expected type. You can use them to ensure that a variable has a certain type before using it. You can also provide a custom error message as an optional second parameter.

import { assertNumber } from "type-guardians/number";

function double(value: unknown) {
  assertNumber(value);
  // value is now of type number
  return value * 2;
}

API

String

• isString(value: unknown): value is string
• assertString(value: unknown, message?: string): asserts value is string

Number

• isNumber(value: unknown): value is number
• assertNumber(value: unknown, message?: string): asserts value is number

Error

• isError(value: unknown): value is Error
• assertError(value: unknown, message?: string): asserts value is Error