CODEWITHSUNDEEP
typescript
Majeed Siddiqui
Majeed Siddiqui

Reputation: 548

Typescript compiler not showing error

According to Typescript documentation when using object literal it should exactly match interface. But for the following code playground does not show any error message:

code pic from playground

Here prop is string so it conflicts with [index: number]: number which means the index should be a number and value at that index should be a number, for properties other than name and age.

Is it a bug? If I am wrong please explain how this is working?

Upvotes: 2

Views: 495

Answers (2)

Sebastian Sebald
Sebastian Sebald

Reputation: 16846

Because they decided to do "implicit index signatures", which is a good thing! You can read about the reasons here:

https://github.com/Microsoft/TypeScript/issues/7059

But the TypeScript compiler will show an error if you create a property that has a number as key and a non-number as value. Here is an example in the playground.

On a side note: TypeScript tries to be liberate where it can. You can omit parameters from a function signature. See this example.

Upvotes: 0

Pelle Jacobs
Pelle Jacobs

Reputation: 2579

You can always specify more properties than the interface requires. As demonstration, have a look at this code: (or on the playground) 

// index.ts
interface MyInterface {
  obligatoryProperty: string
  optionalProperty?: string
  [index: number]: number  
}

let impl1: MyInterface = {} // fails compilation
let impl2: MyInterface = { obligatoryProperty: 'hello' } // compiles fine
let impl3: MyInterface = {
  obligatoryProperty: 'hello',
  optionalProperty: 'hello',
  2: 2,
  3: 3,
  notSpecifiedPropertyThatIsAlsoNotANumber: 'hello',
} // Still fine

Upvotes: 1

Related Questions