implicitly has an 'any' type because expression of type in typescript

Question

I have a class like this:

interface UserProps {
  name: string
  age: number
}

export class User {
  constructor(private data: UserProps) {}

  get(propName: string): number | string {
    return this.data[propName]
  }
}

and I don't know why I get this error: "Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'UserProps'.
No index signature with a parameter of type 'string' was found on type 'UserProps'"

any advice?

It means that you never check what propName is but you're using it to access a property from UserProps. So if propName = "foo" TS has no way of knowing what would be returned from this.data[propName]. Therefore, it implicitly assigns it any. Element implicitly has an 'any' type because expression of type 'string' can't be used to index — VLAZ
– VLAZ, Commented Aug 24, 2021 at 10:42

Guerric P · Accepted Answer · 2021-08-24 10:53:12Z

1

Your error occurs because you can't use any string as the key, only a subset (name and age). Set keyof UserProps as the type for propName:

interface UserProps {
  name: string
  age: number
}

export class User {
  constructor(private data: UserProps) {}

  get<T extends keyof UserProps>(propName: T): UserProps[T] {
    return this.data[propName]
  }
}

edited Aug 24, 2021 at 10:53

answered Aug 24, 2021 at 10:41

Guerric P

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3 Comments

VLAZ Over a year ago

This isn't enough of an improvement, IMO - user.get("age") still returns number | string which is misleading, at the very least.

Ehsan Over a year ago

@VLAZ I think this is a good solution however I think this error should not happen.

VLAZ Over a year ago

@ehsan it's your code that is sloppy. For it to work, TS also has to be sloppy. Which causes an error. The solution is not good enough because the resulting type doesn't necessarily match what it should be. That's what I was highlighting.

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