strange JS syntax, what does it mean?

Question

I recently came across this strange JavaScript code:

export type ImageSource = {
  id: string,
  URI: string,
  thumbnail: string,
  title: ?string,
  description: ?string
}

type Props = {
  images: Array<ImageSource>,
  onPressImage?: Function,
  topMargin?: number,
  closeText?: string,
  infoTitleStyles?: Animated.View.style,
  infoDescriptionStyles?: Animated.View.style,
  enableTilt?: boolean
}

If I understand it correctly, string is a data type, but can someone explain, what is the difference between string and ?string.

Also, what does the following line of code mean:

topMargin?: number,

I've got it from a sample react-native app, I'm assuming, it signifies an optional property, but I can't find any explanation when googling for it.

It looks like typescript and it means that the field is optional, effectively: number or undefined typescriptlang.org/docs/handbook/… — zerkms
– zerkms, Commented Sep 5, 2018 at 2:28

illiteratewriter · Accepted Answer · 2018-09-05 03:22:35Z

1

This is most probably either Typescript or Flow. topMargin?: number represents an optional parameter and title: ?string represents maybe types.

answered Sep 5, 2018 at 3:22

illiteratewriter

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2 Comments

dmitryame Over a year ago

Wondering, if this is a common thing, why it's not being referred to in any of the react-native documentation?

illiteratewriter Over a year ago

TypeScript is a strongly typed, object oriented, compiled language. Mostly people code in Javascript, and if it was flow; flow is a static typechecker for javascript, sort of like a plugin, so it doesn't make sense to add it to documentation.

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