iskit

Tiny, type-safe predicates — isString, isNumber, isPlainObject, isEmpty, and more, all narrowing TypeScript type guards. Zero dependencies.

A bag of is* checks you write in every project — but done once, correctly, with real type-guard signatures so each check narrows the type. isNumber rejects NaN, isPlainObject actually means plain, and isEmpty only calls a value empty if it's a container. Zero dependencies, tree-shakeable.

import { isString, isDefined, isEmpty } from "@billdaddy/iskit";

function f(x: unknown) {
  if (isString(x)) x.toUpperCase(); // x is `string`
}

users.filter(isDefined);            // drops null/undefined, type is User[]
if (isEmpty(input)) return;         // "", [], {}, Map/Set(0) → true

Why iskit?

• Everything narrows. Each guard carries a value is T signature — no casts, works in if, filter, ternaries.
• Sharper than typeof. isNumber excludes NaN; isDate rejects invalid dates; isPlainObject excludes arrays, Map/Set, and class instances.
• Sensible isEmpty. Empty means an empty container (string, array, object, Map/Set) or nullish — not every non-collection the way lodash does.
• Non-empty narrowing. isNonEmptyArray narrows to [T, ...T[]], so arr[0] loses its | undefined.
• Tree-shakeable. sideEffects: false, ESM + CJS, zero dependencies.

Install

npm install @billdaddy/iskit
# or: pnpm add @billdaddy/iskit  /  yarn add @billdaddy/iskit  /  bun add @billdaddy/iskit

Primitives

import {
  isString, isNumber, isInteger, isBoolean, isBigInt, isSymbol,
  isFunction, isNull, isUndefined, isNil, isDefined, isPrimitive,
} from "@billdaddy/iskit";

isNumber(NaN);    // false  (a pass is always a usable number)
isNumber(42);     // true
isNil(null);      // true   (null | undefined)
isDefined(0);     // true   (narrows away null | undefined)
isPrimitive([]);  // false

Objects

import {
  isObject, isArray, isPlainObject, isDate, isRegExp,
  isMap, isSet, isError, isPromise,
} from "@billdaddy/iskit";

isPlainObject({});            // true
isPlainObject(new Date());    // false
isPlainObject(new Foo());     // false (class instance)
isDate(new Date("nope"));     // false (invalid time)
isPromise({ then() {} });     // true  (thenables count)

Emptiness

import { isEmpty, isNonEmptyString, isNonEmptyArray } from "@billdaddy/iskit";

isEmpty("");          // true
isEmpty([]);          // true
isEmpty({});          // true
isEmpty(new Set());   // true
isEmpty(0);           // false  ← not a container
isEmpty({ a: 1 });    // false

isNonEmptyString(s);  // narrows to string
isNonEmptyArray(xs);  // narrows to [T, ...T[]]

Pairs well with

| Need | Use | | --- | --- | | Assert a condition and narrow / throw | assertkit | | Deep equality / clone | equalkit |

Contributors ✨

This project follows the all-contributors specification. Contributions of any kind are welcome — code, docs, bug reports, ideas, reviews! See the emoji key for how each contribution is recognized, and open a PR or issue to get involved.

Thanks goes to these wonderful people:

License

MIT © Tung Tran