How to declare default-function for a class?

Question

The Function Call Operator in C++ allows a class to act like it were a function. I have used it in a logging class where something like this:

logger.setFileName("./debug.log");
logger.log("Log this message");

turns into this:

logger.setFileName("./debug.log");
logger("Log this message");

And now I would like to use Function Call Operator or something in that direction in type-script (to provide a default-function for my class), but how do we achieve such a declaration in type-script?

Should we rather make a feature request at GitHub?

Answer 1

This would need to be a JavaScript proposal.

You can't make a JavaScript class that behaves like this, but you can make a regular function that acts like that, though it takes some gymnastics:

interface Logger {
  (message: string): void;
  setFileName(filename: string): void;
}

const logger = function (message: string) {

} as Logger;

logger.setFileName = function (filename: string) {

}

logger.setFileName("./debug.log");
logger("Log this message");

Edit: here's an example that uses a createLogger() factory function to create multiple instances with state stored in function scope:

interface Logger {
  (message: string): void;
  setFileName(filename: string): void;
}

const createLogger = (): Logger => {
  let _filename: string;
  const logger = function (message: string) {
    console.log(_filename, message);
  } as Logger;

  logger.setFileName = function (filename: string) {
    _filename = filename;
  }

  return logger;
}

const logger1 = createLogger();
const logger2 = createLogger();
logger1.setFileName("./debug1.log");
logger2.setFileName("./debug2.log");
logger1("one");
logger2("two");

// Output:
// ./debug1.log one
// ./debug2.log two