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Anton Selin
Anton Selin

Reputation: 3642

TypeScript Optional function in Interface

Is it possible to create an Interface in TypeScript with optional function?

interface IElement {
  name: string;
  options: any;
  type: string;
  value?: string;
  validation(any): boolean; // --> should be optional.

}

Upvotes: 183

Views: 95163

Answers (2)

Shubham Verma
Shubham Verma

Reputation: 9971

Just like field properties, we can also declare optional methods in an interface, just by placing "?" after the method name

 interface Vehicle{
  run?(): void;
}

class Sedan implements Vehicle {}

class SUV implements Vehicle {
  run() {
    console.log("run run");
  }
}

let suv = new SUV();

suv.run();

Upvotes: 6

iCollect.it Ltd
iCollect.it Ltd

Reputation: 93631

There are currently three syntaxes that TypeScript allows for function declarations in interfaces:

Using your example of a validation function taking 1 parameter (of any type) and a boolean return value:

validation: {(flag: any): boolean};

or in the newer syntax:

validation(flag: any) : boolean;

or an alternative is:

validation: (flag: any) => boolean;

Solution:

so to make it optional with the old syntax is easy:

validation?: {(flag: any): boolean};

with the second syntax (recent addition - thanks to @toothbrush)

validation?(flag: any) : boolean;

or in the third syntax (as you found):

validation?: (flag: any) => boolean;

Upvotes: 324

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