[TypeScript] Define a function type

type DigitValidator = (char) => boolean;

const numericValidator = (char) => /[0-9]{1}/.test(char);

export const digitValidators: {[key: string]: DigitValidator} = {
  '9': numericValidator
};

We can use 'type' keyword to define a function type. 

'digitValidators', is a mapping object, return a function which type is DigitValidator.