Posted on Apr 22, 2020 • Edited on Apr 23, 2020

JavaScript error handling pattern tip

Don't throw a new Error(msg) if you not sure you handle it everywhere you use a module. It can break your app and it's not ideal UX for a user. You can follow the Either pattern from FP to avoid such situations.

// success and error cases Success: { _tag: "Right", right: "Success data" } Error: { _tag: "Left", left: "Error message" } // helpers to define right or left value export const isLeft = (left) => { switch (left._tag) { case 'Left': return true case 'Right': return false } } export const isRight = (right) => !isLeft(right) export const right = (right) => ({ _tag: "Right", right }) export const left = (left) => ({ _tag: "Left", left }) // simple usage import * as E from 'helpers/either' const success = E.right("some value") const error = E.isLeft("some error") console.log(isRight(success) && success.right) // "some value" console.log(isLeft(error) && error.left) // "some error"

Top comments (2)

Heiker • Apr 23 '20

If you're going to use tags and helper functions, why not go for a little pattern matching too?

function match(value, patterns) { const { type = null } = value || {}; const _match = patterns[type]; if(typeof _match == 'function') { return _match(value.data); } else if (typeof patterns._ == 'function') { return patterns._(); } else { return; } } function Union(types) { return types.reduce((target, type) => { target[type] = (data) => ({ type, data, match: patterns => match({type, data}, patterns) }); return target; }, {}); }

Then you can have this.

const Result = Union(['Ok', 'Err']); const something = Result.Ok({ stuff: 'good stuff' }); const maybe = Result.Err('this is still data'); something.match({ Ok: ({stuff}) => `the ${stuff}`, Err: (msg) => msg, _: () => 'What?' }); // the good stuff

For a better version of that checkout this library.

Max Frolov • Apr 23 '20

Thanks for the suggestion, nice lib :) I use FP-TS and the pattern above is an abstract demonstration of the way of thinking