When using TypeScript, a typed interface will not permit a value that is not within its specified string literal type

I have downsized my issue to a smaller scale.

This class needs to set the default value of its "status" property. The type T extends the string literal type "PossibleStatus" which consists of 3 possible strings.

Typescript is giving me trouble with this. Can you assist in deciphering why?

Visit this stackblitz for a live demonstration

export type PossibleStatuses = 'idle' | 'on' | 'off';

export class StatefulNode<T extends PossibleStatuses> {

  private status: T = 'idle';

  constructor() { }

}
typescriptgenerics

Answer №1

Here is an example that highlights the reason for inability to initialize status (type X = 'on' extends PossibleStatuses but excludes 'idle'): 

export type PossibleStatuses = 'idle' | 'on' | 'off';

export class StatefulNode<T extends PossibleStatuses> {
  // Error
  private status: T = 'idle';
}

// This is because
const unsafe = new StatefulNode<'on'>();

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