What is the point of StyleSheet.create - react-native

Question

I'm reading the React Native docs / tutorial, and I'm wondering what the point of the StyleSheet.create function is.
For example, the tutorial has the following code:
const styles = StyleSheet.create({
bigblue: {
color: 'blue',
fontWeight: 'bold',
fontSize: 30,
},
red: {
color: 'red',
},
});
But I don't understand the difference between that and:
const styles = {
bigblue: {
color: 'blue',
fontWeight: 'bold',
fontSize: 30,
},
red: {
color: 'red',
},
};

Answer 1

TL;DR Always use StyleSheet.create() when you can.
The answer by Nico is correct, but there is more to it.
To summarize:
It validates the styles as mentioned by Nico
As mentioned in the documentation:
Making a stylesheet from a style object makes it possible to refer to it by ID instead of creating a new style object every time.
Also mentioned in the documentation:
It also allows to send the style only once through the bridge. All subsequent uses are going to refer an id (not implemented yet).
As you might know, sending the data across the bridge is a very costly operation that has significant impact on the performance of the application. So, using StyleSheet.create() you reduce the strain on the bridge.

Answer 2

StyleSheet.create does not add performance gains anymore.
https://github.com/DefinitelyTyped/DefinitelyTyped/issues/29265#issuecomment-430783289
quoting the github comment:
#alloy I understand what docs says, but can prove that code:
const myStyle: ViewStyle = { flex: 1 } export const FlexView:
React.SFC = (props) => <View style={myStyle}>{props.children}</View>
has almost same performance (even slightly faster) compared to
const s = StyleSheet.create({ flex: { flex: 1 } })
export const FlexView: React.SFC = (props) => <View style={s.flex}>{props.children}</View>
because if you look at sources, you discover that latest chunk effectively extracted to this (see:
https://github.com/facebook/react-native/blob/0.57-stable/Libraries/StyleSheet/StyleSheet.js#L373):
const s = { flex: { flex: 1 } }
export const FlexView = (props) => <View style={s.flex}>{props.children}</View>
And yes, in previous
versions of RN it was global registry of styles, but it was even more
slow, because it never crossed bridge border actually (proof from 0.55
branch) 😀

Answer 3

Here is there source code of create.
create<T: Object, U>(obj: T): {[key:$Keys<T>]: number} {
var result: T = (({}: any): T);
for (var key in obj) {
StyleSheetValidation.validateStyle(key, obj);
result[key] = ReactNativePropRegistry.register(obj[key]);
}
return result;
}
I am not an expert of React in any. I actually never used it but here are my insights. It seems that create does some kind of validation over your keys and register them to React.
I think you could skip the validation by simply not calling create but I'm not sure what ReactNativePropRegistry.register does exactly.
Reference to the source

Answer 4

As #Mentor pointed out in the comments:
.create still only validates in development and does nothing else. In production it just returns the object. See source code in repository.
source code
create<+S: ____Styles_Internal>(obj: S): $ObjMap<S, (Object) => any> {
if (__DEV__) {
for (const key in obj) {
StyleSheetValidation.validateStyle(key, obj);
if (obj[key]) {
Object.freeze(obj[key]);
}
}
}
return obj;
}
I think this comment deserves to be more noticeable. So I post it as an answer.
Additionally, I'd like to point out that validation - is a good thing, but there is another, better way to validate - Typescript:
const styles = StyleSheet.create({
someViewStyle: { ... },
someTextStyle: { ... },
})
can be replaced with
import { ..., ViewStyle, TextStyle } from 'react-native';
interface Styles {
someViewStyle: ViewStyle,
someTextStyle: TextStyle,
}
const styles = {
someViewStyle: { ... },
someTextStyle: { ... },
}
And it's not just static-time check, it also allows to discriminate between ViewStyle and TextStyle.
But there are more lines of code. So, personally, I prefer to go without styles object, if possible:
const someViewStyle: ViewStyle = { ... },
const someTextStyle: TextStyle = { ... },