Mastering TypeScript 3 - Third Edition

Interface function definitions

Interfaces, like classes, follow the same rules when dealing with functions. To update our IComplexType interface definition to match the ComplexType class definition, we need to write a function definition for each of the new functions, as follows:

interface IComplexType { 
    id: number; 
    name: string; 
    print(): string; 
    usingTheAnyKeyword(arg1: any): any; 
    usingOptionalParameters(optionalArg1?: number) : void; 
    usingDefaultParameters(defaultArg1?: number) : void; 
    usingRestSyntax(...argArray: number []) : void; 
    usingFunctionCallbacks(callback: (id: number) => string); 
} 

This interface definition includes the id and name properties and a print function. We then have a function signature for the usingTheAnyKeyword