TypeScript: Utilizing specific objects as function arguments in implementing an interface

Question

TypeScript: Utilizing specific objects as function arguments in implementing an interface

To better understand this concept, let's take a closer look at the code snippet provided:

interface FooInterface {
  bar: (flags: { [key: string]: string }) => void;
}

export class Foo implements FooInterface {
  bar(flags: { myFlag: string }) {}
}

In implementing FooInterface.bar, I require that the parameter be an object, regardless of its keys.

However, when I implemented this in the Foo class with the key named as myFlag, it resulted in an error stating that this particular key does not exist within the interface. The full error message is shown below.

How can I instruct Typescript to overlook the specific keys used in the implementation classes?

The error received:

src/extensions/test/test.provider.ts:24:3 - error TS2416: Property 'bar' in type 'Foo' cannot be assigned to the same property in base type 'FooInterface'.
  Type '(flags: { myFlag: string; }) => void' is not compatible with type '(flags: { [key: string]: string; }) => void'.
    The parameters 'flags' are incompatible.
      Property 'myFlag' is required in type '{ myFlag: string; }', but missing in type '{ [key: string]: string; }'.

24   bar(flags: { myFlag: string }) {}
     ~~~

typescript object interface

Answer 1

Answer №1

To ensure the flags are of type object with string values using generic typings, you can define the type in the class implementation like this:

interface FooInterface<T extends { [key: string]: string }> {
  bar: (flags: T) => void;
}

type BarFlags = { myFlag: string };

export class Foo implements FooInterface<BarFlags> {
  bar(flags: BarFlags) {}
}

Try it out here

Answer 2

To ensure the flags are of type object with string values using generic typings, you can define the type in the class implementation like this:

interface FooInterface<T extends { [key: string]: string }> {
  bar: (flags: T) => void;
}

type BarFlags = { myFlag: string };

export class Foo implements FooInterface<BarFlags> {
  bar(flags: BarFlags) {}
}

Try it out here

Answer 3

Answer №2

The issue at hand is that the requirement for myFlag to be a string contradicts with the type definition of { [key: string]: string }, as it does not ensure the presence of the myflag key. Therefore, it cannot fully adhere to the string type.

If you designate the myFlag key as optional, the problem can be resolved by verifying its existence before use.

interface FooInterface {
  bar: (flags: { [key: string]: string }) => void;
}

export class Foo implements FooInterface {
  bar(flags: { myFlag?: string }) {
    if (flags.myFlag) {
      console.log(flags.myFlag) // outputs a string
    }
  }
}

Code Playground

If ensuring that myFlag is provided when calling bar in the Foo class is necessary, then @leonardfactory's solution is the appropriate approach.

Answer 4

The issue at hand is that the requirement for myFlag to be a string contradicts with the type definition of { [key: string]: string }, as it does not ensure the presence of the myflag key. Therefore, it cannot fully adhere to the string type.

If you designate the myFlag key as optional, the problem can be resolved by verifying its existence before use.

interface FooInterface {
  bar: (flags: { [key: string]: string }) => void;
}

export class Foo implements FooInterface {
  bar(flags: { myFlag?: string }) {
    if (flags.myFlag) {
      console.log(flags.myFlag) // outputs a string
    }
  }
}

Code Playground

If ensuring that myFlag is provided when calling bar in the Foo class is necessary, then @leonardfactory's solution is the appropriate approach.

TypeScript: Utilizing specific objects as function arguments in implementing an interface

Answer №1

Answer №2

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