Typescript, interfaces and variable declaration: what does it means?

Question

This is part of a Material-UI interface:

export interface DialogProps
  extends StandardProps<ModalProps & Partial<TransitionHandlerProps>, DialogClassKey, 'children'> {
  /**
   * The id(s) of the element(s) that describe the dialog.
   */      
  /**
   * If `true`, the dialog stretches to `maxWidth`.
   *
   * Notice that the dialog width grow is limited by the default margin.
   */
  fullWidth?: boolean;
  /**
   * Determine the max-width of the dialog.
   * The dialog width grows with the size of the screen.
   * Set to `false` to disable `maxWidth`.
   */
  maxWidth?: 'xs' | 'sm' | 'md' | 'lg' | 'xl' | false;
  /**
   * Callback fired when the backdrop is clicked.
   */      
  /**
   * Props applied to the [`Transition`](http://reactcommunity.org/react-transition-group/transition#Transition-props) element.
   */
  TransitionProps?: TransitionProps;
}

I tried the following assignment (it works!):

const maxWidth: DialogProps['maxWidth'] = 'lg'

My question is: what does it means? Could I consider it as declaring a new variable that is a 'sub-type' of DialogProps? Where could I find some documentation about this topic in typescript?

Thank you

user13258211 · Accepted Answer · 2020-04-29 10:30:38Z

You can retrieve the type of a property when you access it like this: DialogProps['maxWidth']

export interface DialogProps {
  /**
   * The id(s) of the element(s) that describe the dialog.
   */      
  /**
   * If `true`, the dialog stretches to `maxWidth`.
   *
   * Notice that the dialog width grow is limited by the default margin.
   */
  fullWidth: boolean;
  /**
   * Determine the max-width of the dialog.
   * The dialog width grows with the size of the screen.
   * Set to `false` to disable `maxWidth`.
   */
  maxWidth: 'xs' | 'sm' | 'md' | 'lg' | 'xl' | false;     
}


const maxWidth: DialogProps.maxWidth = 'lg' 

console.log(maxWidth);

If you hover the red underlined text the compiler actually tells you that you can retrieve the type of a property using [propertyName].

Didn't know that myself, documentation is here: https://github.com/Microsoft/TypeScript/blob/1db4f96fd14fc3de5ae3704e925afd6474cfb8f5/doc/spec.md#4.13

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