Typescript interface default values

Question

I have the following interface in TypeScript:

interface IX {
    a: string,
    b: any,
    c: AnotherType
}

I declare a variable of that t

Answer 1

You could use two separate configs. One as the input with optional properties (that will have default values), and another with only the required properties. This can be made convenient with & and Required:

interface DefaultedFuncConfig {
  b?: boolean;
}

interface MandatoryFuncConfig {
  a: boolean;
}

export type FuncConfig = MandatoryFuncConfig & DefaultedFuncConfig;
 
export const func = (config: FuncConfig): Required => ({
  b: true,
  ...config
});

// will compile
func({ a: true });
func({ a: true, b: true });

// will error
func({ b: true });
func({});