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Predicates Background

Predicates Background is a library of conditional checks (predicates) with strong typing using type predicates.


There are lots of checks I did in every project (falsy checks, type checks, matches, includes, etc.) so I wanted to have all of them in a single place and this library was born out of that necessity.


This library relies on a type narrowing feature of TypeScript called type predicates. This feature allows to make functions with a shape like this:

const isString = (value: unknown): value is string => typeof value === "string";

Affecting the type narrowing in typescript like this:

if (isString(value)) {
// value is considered string here

I built this library using TypeScript, with configurations from my shared configs. I made it ESM only as soon as Node started supporting ESM modules, and I made it tree-shakeable by using named exports.

Currying is used extensively. For example the util isType is used by isFunction, isString, and isNumber to name a few. isType internally uses typeof and a dictionary of types, but is a little bit different to using typeof directly because null returns "null" instead of "object".


As I mentioned in previous sections, I used to have this checks all over, so when I moved that code here, it turned into a pretty common dependency in my libraries and at work. It is used by cron, iterables, notify, utils, and more.