Type guards

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

Each one narrows the type.

• Usage
• The guards
  • isNull
  • isDefined
  • isString
  • isNumber
  • isBoolean
  • isFunction
  • isObject
• What is not here

Usage

import { isDefined, isObject, isString } from '@studiometa/js-toolkit/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.

The guards

isNull

isNull(value: unknown): value is null

isDefined

isDefined<T>(value: T | undefined): value is T

Narrows T | undefined to T. Passing it by reference to filter is what it is for.

isString

isString(value: unknown): value is string

isNumber

isNumber(value: unknown): value is number

It rejects NaN, because a NaN that passes a number check is a bug that surfaces three functions later.

isBoolean

isBoolean(value: unknown): value is boolean

isFunction

isFunction(value: unknown): value is (...args: unknown[]) => unknown

isObject

isObject(value: unknown): value is Record<string, unknown>

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.