react-is 19.x

Complete instructions

Verified against react-is@19.2.8 on 2026-09-08. 2 of 2 examples executed.

react-is 19

react-is provides brand checks for arbitrary values and React element types. Import the package namespace:

import * as ReactIs from "react-is";

React 19 exports

React 19 exposes these validation functions:

• isValidElementType
• isElement
• isFragment
• isPortal
• isStrictMode
• isContextConsumer
• isContextProvider
• isForwardRef
• isLazy
• isMemo
• isProfiler
• isSuspense
• isSuspenseList

It also exposes typeOf, which returns the React type brand for a React-created value, and these type-brand constants:

• Element
• Fragment
• Portal
• StrictMode
• ContextConsumer
• ContextProvider
• ForwardRef
• Lazy
• Memo
• Profiler
• Suspense
• SuspenseList

Validate component types

isValidElementType accepts a host element name such as "div" and component functions. It is a check for a value that React can use as an element type, not a check that a value is already an element.

import * as assert from "node:assert/strict";
import * as ReactIs from "react-is";

function Component() {
  return null;
}

assert.equal(ReactIs.isValidElementType("div"), true);
assert.equal(ReactIs.isValidElementType(Component), true);
assert.equal(ReactIs.isValidElementType(null), false);
assert.equal(ReactIs.isValidElementType({}), false);

Check arbitrary values without React or JSX

All element predicates safely reject unrelated values. This makes them usable in a plain script, although a positive result generally requires a value created by React or ReactDOM.

import * as assert from "node:assert/strict";
import * as ReactIs from "react-is";

const value = {};

assert.equal(ReactIs.isElement(value), false);
assert.equal(ReactIs.isFragment(value), false);
assert.equal(ReactIs.isPortal(value), false);
assert.equal(ReactIs.isStrictMode(value), false);
assert.equal(ReactIs.isContextConsumer(value), false);
assert.equal(ReactIs.isContextProvider(value), false);
assert.equal(ReactIs.isForwardRef(value), false);
assert.equal(ReactIs.isLazy(value), false);
assert.equal(ReactIs.isMemo(value), false);
assert.equal(ReactIs.isProfiler(value), false);
assert.equal(ReactIs.isSuspense(value), false);
assert.equal(ReactIs.isSuspenseList(value), false);

React-created values

With React available, use typeOf and the predicates against values created by React:

• typeOf(<div />) === Element and isElement(<div />) identify an element.
• typeOf(<></>) === Fragment and isFragment(<></>) identify a fragment.
• isStrictMode(<React.StrictMode />) and typeOf(...) === StrictMode identify strict mode.
• isContextConsumer(<ThemeContext.Consumer />) and isContextProvider(<ThemeContext.Provider />) identify context values. Their brands are ContextConsumer and ContextProvider.

These forms require React and JSX (or equivalent calls to React APIs), so they are not standalone bun example.ts examples under this skill's execution constraints.

Portals

Portal checks require both ReactDOM and a DOM container:

const div = document.createElement("div");
const portal = ReactDOM.createPortal(<div />, div);
ReactIs.isPortal(portal); // true
ReactIs.typeOf(portal) === ReactIs.Portal; // true

This requires a browser-like document, ReactDOM, and a React-created element; it cannot run in the standalone no-DOM example environment.

React 18 compatibility trap

Do not copy the older React 18 API shape into React 19 code:

• React 19 does not export isAsyncMode or isConcurrentMode.
• The corresponding AsyncMode and ConcurrentMode aliases from the older implementation are not part of the React 19 stable export list.
• React 18 documentation showed isValidElementType(React.createFactory("div")); that example is not included in the React 19 documentation.

React 19 also has feature-aware handling for context consumer and provider brands. Use the exported ContextConsumer and ContextProvider constants and the matching predicates rather than assuming the older internal context symbols.

TypeScript

The researched package metadata identifies @types/react-is as the separately published TypeScript declaration package. The package itself is not documented in the researched sources as shipping TypeScript declarations.

Not covered

This skill does not cover React rendering, JSX compilation, ReactDOM setup, browser DOM setup, creation of portals, creation of contexts, or construction of forward-ref, lazy, memo, profiler, suspense, or suspense-list values. It also does not cover undocumented internal symbol shapes, bundler configuration, or the separately published declaration package beyond noting its existence.