使用Object.keys将数组错误映射到字符串（javascript）

Question

I have an object of type c.errors that returns, in a specific case:

{ required: true}

And I want to map this against the more general (and fixed array)

   validationMessages = {
    required: 'Please enter',
    pattern: 'Please enter valid'
  }

I also have an errorMessage property initialised with an empty string . My attempt was:

 this.errorMessage = Object.keys((c.errors).map(key => this.validationMessages[key]).join(' '));

Errors I get:

Parameter 'key' implicitly has an 'any' type.

Element implicitly has an 'any' type because type '{ required: string; pattern: string; }' has no index signature.

Any advice?

this.errorMessage should contain 'Please enter' (that's my expectation after mapping)

Answer 1

If you index into a type that does not have an indexer, typescript expects the string you index by to be a key of the type. This means that in you case, key should be either string literal type 'required' or 'pattern' . Since Object.keys returns an array of strings , if you are certain that array will only contain valid keys for validationMessages you can just use a type assertion. Also you have some mismatched parenthesis:

this.errorMessage = Object.keys(c.errors).map((key) => this.validationMessages[key as 'required']).join(' ');