Disabling font scaling can hurt the accessibility of your app, ideally if you want to limit scaling for Apps using React native 0.58.0 and above; use the maxFontSizeMultiplier prop on specific Text components.
However if you absolutely want to disable font scaling across your entire Application, you can do so by globally setting the allowFontScaling prop in the defaultProps of Text.
You should place these lines in your root entrypoint (normally index.js) before AppRegistry.registerComponent.
For React Native 0.56.0+
Text.defaultProps = Text.defaultProps || {};
Text.defaultProps.allowFontScaling = false;
For earlier versions of React Native you should only need the second line, but having both won’t hurt. The first line just protects against the Text component not having defaultProps which is the case for React Native 0.56.0 and above.
Add the above lines in the entry point file of your React Native application (usually index.js, app.js or main.js) to apply this prop to all Text components in your application.
This prop will only affect Text components and you may want to apply the same changes to TextInput which can be done with a similar snippet:
TextInput.defaultProps = TextInput.defaultProps || {};
TextInput.defaultProps.allowFontScaling = false;
Also note that some components wont obey font scaling settings, for example: Alert, PickerIOS, DatePickerIOS, TabBarIOS, SegmentedControlIOS as these are all natively drawn and don’t rely on the Text component.