CODEWITHSUNDEEP
typescriptecmascript-6
Norayr Ghukasyan
Norayr Ghukasyan

Reputation: 1408

Types are incompatible when using object spread

I am facing a strange error in typescript, which I couldn't find on the internet.

type IDialogTypeProps = 'info' | 'alert' | 'success' | 'warning';

  interface IDialogProps {
    text: string;
    type: IDialogTypeProps;
  }

  const obj = {type: 'warning'}

const dialog:IDialogProps = {
  text: 'string',
  ...obj,
}

Typescript throughs a warning about that 

Types of property 'type' are incompatible.
Type 'string' is not assignable to type 'IDialogTypeProps'

But when if I put type: string directly in the dialog object, typescript will compile without warnings. I am sure this is a pretty easy issue to solve.

Upvotes: 1

Views: 364

Answers (1)

CertainPerformance
CertainPerformance

Reputation: 370609

The problem is that your obj object, without being given an explicit type, is assumed by Typescript to be:

const obj: {
  type: string;
}

You can force TS not to automatically widen the inferred type from 'warning' to string by denoting the 'warning' string as const. Change obj's definition to:

const obj = { type: 'warning' as const }

Upvotes: 2

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