Possibly bug, TS wrongly complains that generic property is null

[al6x]

TS v 3.3.3, enable strictNullChecks

Code

// Errors is a collection of form errors
//
// const errors = { 
//   title: ["can't be blank", "invalid characters"],
//   text:  ["too long"],
// }
//
// It could be rendered as
// 
// <FormInput name="title" errors={errors}/>


// Requiring that Errors had field of type T
type Errors<T extends string> = { [k in T]?: string[] }

export function FormInput<T extends string>(
  { name, errors }: { name: T, errors: Errors<T> }
) {
  // TS complains that there's no length
  const hasErrors = (errors[name] || []).length > 0
}

Expected behavior:

Should correctly infer that (errors[name] || []) is an Array.

Actual behavior:

Wrongly complains about null

Playground Link:

Play

[RyanCavanaugh]

Duplicate #22137 - we don't have type system machinery in place yet to narrow away undefined/null from types originating in a generic type.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.