Javascript addeventlistener working without parameters

I was looking for javascript documentation and found this

const log = document.getElementById('log');

document.addEventListener('keydown', logKey);

function logKey(e) {
  log.textContent += ` ${e.code}`;
}

I don’t understand how logkey function is working in addeventlistener . when I press a key, the console prints its code but logkey is not having any parameters in addeventlistener. How did it print e.code ?

Answer

You pass the function as a parameter to the eventListener. The eventListener calls the logKey function with the event as a function parameter.

See in the following example how a parameter is passed on to the function:

function func1(function_as_parameter){
  function_as_parameter('Shivam');
}

function helloWordFunction(name){
  console.log(`Hello ${name}!`);
}

func1(helloWordFunction); // Hello Shivam!