Callback Function with parameters in JS

function greetUser(name, callback) {
  //console.log("Hi " + name);
  const num = '66'
  const num2 = '67'
  
  callback(num);
}



greetUser("Alice", (param, param2) => {
  console.log(param);
});

here the confusion ‘how the param or param2 in callback func getting the value of num. it is actually JS IFFE concept.

function greetUser(name, callback) {
  //console.log("Hi " + name);
  const num = '66'
  const num2 = '67'
  
  callback(num); // here callback is becoming this
        ↓  
  (param, param2) => { console.log(param); } (num);

  // and here (num) is actually agrumnent and param/param2 is parameteres. num 
  // is passed to param. 

}

now lets see this is express

app.get('/', (req, res) => {
  res.send('Hello World!');
});

so here req and res are actually parameter, these were given to use req,res’s methods like .send .

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const res/req = //something;

so whole res/req thing can be used. i think error works in same way in try/catch.