isDefined()

Category

Utilities

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SSR

SSR support

Last updated: 24/04/2026

Overview

isDefined is a type guard value is T for value: T | null | undefined: it returns true when the value is neither null nor undefined, and false otherwise, allowing TypeScript to narrow in if, ternary, and .filter(isDefined) on arrays without a wider return type. It is a one-line check-no deep** falsy handling (0, empty string, NaN are still “defined”). It often pairs with optional API fields or partial records when you need a typed list of “present” values.

What it accepts

1. value: T | null | undefined

What it returns

• boolean, narrows to T when true

Usage

Filter a list of ids (some missing); map to length only for defined strings.

import isDefined from '@dedalik/use-react/utils/isDefined'

function Example() {
  const ids = ['a', null, 'bb', undefined, 'ccc'] as (string | null | undefined)[]

  const lengths = ids.filter(isDefined).map((s) => s.length)

  return (
    <ul>
      {lengths.map((n) => (
        <li key={n}>Length {n}</li>
      ))}
    </ul>
  )
}

export default function Demo() {
  return <Example />
}

API Reference

isDefined

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

Copy-paste hook

TypeScript

/**
 * Type guard for non-null and non-undefined values.
 */
export default function isDefined<T>(value: T | null | undefined): value is T {
  return value !== null && value !== undefined
}

JavaScript

/**
 * Type guard for non-null and non-undefined values.
 */
export default function isDefined(value) {
  return value !== null && value !== undefined
}