Ecmascript 6语法？

Question

I am referring to the implementation of android back button for react native but I am not sure how to interpret this snippet code(for function arguments of addEventListener and removeEventListener . Isn't named parameter supposed to be using = sign?. eg

var foo = function (a = 1) {}

var BackAndroid = {

  exitApp: function() {
    DeviceEventManager.invokeDefaultBackPressHandler();
  },

  addEventListener: function (
    eventName: BackPressEventName,
    handler: Function
  ): {remove: () => void} {
    _backPressSubscriptions.add(handler);
    return {
      remove: () => BackAndroid.removeEventListener(eventName, handler),
    };
  },

  removeEventListener: function(
    eventName: BackPressEventName,
    handler: Function
  ): void {
    _backPressSubscriptions.delete(handler);
  },

};

Answer 1

What you are seeing are default parameters. In that function it's saying if you don't pass a value for a or pass undefined, instead use 1.

Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters