JavaScript

Choose evaluation when you need a value, injection for side effects, or a script prop for each page load.

Evaluate and read a result

const json = await webview.evaluateJavaScript('({ title: document.title })')
const result = JSON.parse(json)
console.log(result.title)

evaluateJavaScript returns JSON text on both platforms. Parse it once. Undefined and null produce the string "null". Use JSON-compatible results.

Page result	Returned text
2	2
'hello'	"hello"
({ a: 1 })	{"a":1}
undefined, null	null

iOS rejects evaluation and serialization errors. Android cannot distinguish a page exception from a null result. JSON whitespace and object key order are not contractual.

Run a side effect

webview.injectJavaScript("document.body.dataset.theme = 'dark'")

This method returns void. It does not report the result, errors, or completion. Do not use it to confirm an operation succeeded.

Run on every navigation

injectedJavaScript runs at document end. injectedJavaScriptBeforeContentLoaded runs in the main frame at document start when the platform supports it.

<NitroWebView
  source={{ uri: 'https://app.example.com' }}
  injectedJavaScriptBeforeContentLoaded="window.APP_PLATFORM = 'native';"
/>

iOS uses a document-start WKUserScript. Android uses DOCUMENT_START_SCRIPT when available. Older Android WebViews fall back to onPageStarted, which does not guarantee execution before the first inline script.

Changing the script prop applies to the next navigation. It does not rerun it in the current page.

Disable JavaScript

javaScriptEnabled={false} disables page JavaScript and the message/injection/history bridge. On iOS this is an initial setting: remount with a new React key to change it. Evaluation rejects when JavaScript is disabled on iOS.