OnError Callback Library For Typescript

Introduction

This TypeScript library provides the type signature for an OnError() callback function.

Use this to delegate error handling to the code that is calling your library.

Quick Start

# run this from your Terminal
npm install @ganbarodigital/ts-on-error

// add this import to your Typescript code
import { OnError } from "@ganbarodigital/ts-on-error/V1"

VS Code users: once you've added a single import anywhere in your project, you'll then be able to auto-import anything else that this library exports.

V1 API

OnError()

export type OnError<E = object, T = never> = (reason: symbol, description: string, extra: E) => T;

OnError() is a function type. It provides a standard function signature to use in error callbacks.

For example:

export const corruptState = Symbol("CORRUPT STATE");

export function doWork(input: object, onError: OnError<object, object>) {
    // a made-up problem
    if (input.property1 === undefined) {
        // tell the error handler what went wrong
        // and give it an opportunity to fix things too
        input = onError(corruptState, "'property1' is missing", input);
    }
}

The idea here is to make it easier to reuse code, by splitting up the error checking and the error handling:

• your library code is responsible for the error checking
• whoever is calling your library code is responsible for the error handling

OnError() takes three parameters:

• reason: symbol is the type of error that has occurred. The calling code will use this to work out what to do with the error.
• description: string is a human-readable explanation of the error. This should be suitable for adding to a log file.
• extra: E is a data bag, containing any extra information relevant to the error. By default, it is an object. We've made it a generic type, so that you can override its type and avoid using type-guards at runtime.

By default, an OnError() handler does not return. It should throw an Error of some kind.

If it's possible for the error handler to recover from the error, you can change the return type from never to whatever suits.

NPM Scripts

npm run clean

Use npm run clean to delete all of the compiled code.

npm run build

Use npm run build to compile the Typescript into plain Javascript. The compiled code is placed into the lib/ folder.

npm run build does not compile the unit test code.

npm run test

Use npm run test to compile and run the unit tests. The compiled code is placed into the lib/ folder.

npm run cover

Use npm run cover to compile the unit tests, run them, and see code coverage metrics.

Metrics are written to the terminal, and are also published as HTML into the coverage/ folder.