Well, you have more than one issue here. First one is your web view needs to be wrapped in a containing View component with flex: 1. Second, injectedJavascript only accepts a string - not a function. Third, it seems you are attempting to use hello as a variable without defining it, or if it is a string than your syntax needs to be something like this: injectedJavascript={'alert("hello")'}. Furthermore, injectedJavascript is already fired when the view loads so you are all good there if that's what you are intending to do. You can inject javascript when the web view starts loading though, using a combination of the props, onLoadStart and injectJavascript, but the implementation is quite different, so that's a different question. Try this code:

import React, { Component } from 'react';
import {
  Platform,
  StyleSheet,
  Text,
  View,
  WebView
} from 'react-native';

export default class App extends Component {

  render() {
    let yourAlert = 'alert("hello")'
    return (
     <View style={{flex: 1}}>
       <WebView
        javaScriptEnabled={true}
        domStorageEnabled={true}
        injectedJavaScript={yourAlert}
        source={{ uri: "https://www.google.com" }} style={{ marginTop: 20  }} />
    </View>
   )
  }
}

Please try this:

<WebView
    ref={c => this._webview = c}
    javaScriptEnabled={true}
    injectedJavaScript={`window.testMessage = "hello world"`}
/>

injectedJavaScript is the prop that injects JS into the web view, once, when it loads. This is what you seem to want.

How injectJavascript is used on the other hand? it's by calling it on the ref of the webview to inject more JS programmatically on the fly. In this case it could look something like this for example:

this.webview.injectJavascript(`window.reactComponent.forceUpdate();true;`)

try

injectedJavaScriptBeforeContentLoaded={injectedJavascript}

https://github.com/react-native-webview/react-native-webview/blob/4d8a76f3691479ef22b55e05c07921af99332395/docs/Guide.md#loading-local-html-files