type padding = [number, number, number, number]

interface IPaddingProps {
  defaultValue?: padding
  className?: string
  disabled?: boolean
  min?: number
  max?: number
  onChange?: (value: number | string) => void
}
interface IFieldSate {
  value: padding
}
export default class FieldPadding extends React.Component<IPaddingProps, IFieldSate> {

  readonly state = {
    value: [0, 0, 0, 0]
  }
  constructor(props: IPaddingProps) {
    super(props)
    if (props.defaultValue) {
      this.state.value = [...props.defaultValue]
    }
  }

}

I get an error at state saying:

Type 'number[]' is missing the following properties from type '[number, number, number, number]': 0, 1, 2, 3?

type padding = [number, number, number, number]

interface IPaddingProps {
  defaultValue?: padding
  className?: string
  disabled?: boolean
  min?: number
  max?: number
  onChange?: (value: number | string) => void
}
interface IFieldSate {
  value: padding
}
export default class FieldPadding extends React.Component<IPaddingProps, IFieldSate> {

  readonly state = {
    value: [0, 0, 0, 0]
  }
  constructor(props: IPaddingProps) {
    super(props)
    if (props.defaultValue) {
      this.state.value = [...props.defaultValue]
    }
  }

}

I get an error at state saying:

Type 'number[]' is missing the following properties from type '[number, number, number, number]': 0, 1, 2, 3?

• javascript
• reactjs
• typescript

• Are you sure the error is occurring in the above code. Because you sure don't seem be to setting numbers 0, 1, ,2, 3 – Shubham Khatri Commented Feb 23, 2019 at 6:13
• 1 @ShubhamKhatri The error message talks about properties, not their values – Bergi Commented Feb 23, 2019 at 14:33

2 Answers 2

Your problem is that state is being inferred as an array instead of as a tuple. There are several ways around this, as detailed in this answer. I'll just present one of them here.

Assuming you are using TypeScript 3.0 or above, you can define the following tuple() helper function which takes any number of arguments and returns a tuple of those arguments:

type Narrowable = string | number | boolean | undefined | null | void | {};
const tuple = <T extends Narrowable[]>(...t: T)=> t;

Then, you can use it inside your FieldPadding class definition:

export default class FieldPadding extends React.Component<IPaddingProps, IFieldSate> {
  readonly state = {
    value: tuple(0, 0, 0, 0) 
  }
}

and that gives value the type [0, 0, 0, 0], a tuple type with four elements of numeric literal type 0.

UPDATE:

So you want state to be readonly but you want to change the values of state.value from 0 to other numbers?