Type guards

import { isDefined, isNumber, isObject, isString } from '@studiometa/js-toolkit-v4/utils';

Each one narrows the type.

Function	Signature
isNull(value)	value is null
isDefined(value)	value is T — for T | undefined
isString(value)	value is string
isNumber(value)	value is number
isBoolean(value)	value is boolean
isFunction(value)	value is (...args: unknown[]) => unknown
isObject(value)	value is Record<string, unknown>

Usage

import { isDefined, isObject, isString } from '@studiometa/js-toolkit-v4/utils';

function label(value: unknown): string {
  if (isString(value)) return value; // string
  if (isObject(value)) return JSON.stringify(value); // Record<string, unknown>
  return '';
}

function first<T>(items: (T | undefined)[]): T[] {
  return items.filter(isDefined); // T[]
}

isDefined as a filter predicate is the case that earns the export: it is the one narrowing TypeScript will not do from a truthiness check.

isNumber rejects NaN

isNumber(NaN); // false

Because a NaN that passes a number check is a bug that surfaces three functions later.

What is not here

Not shipped	Write
isArray	Array.isArray(value)
isEmpty	the check the caller actually means
isEmptyString	value === ''
isDev	your bundler's own flag

A guard earns its place by narrowing something the platform does not, or by being a predicate you pass by reference.