JavaScript Callback

Error-First Callback

Error-First Callback in Node.js - GeeksforGeeks

Error-First Callback in Node.js is a function which either returns an error object or any successful data returned by the function.

1. The first argument of the callback is reserved for an error if it occurs. Then callback(err) is called.
2. The second argument (and the next ones if needed) are for the successful result. Then callback(null, result1, result2…) is called.

Function Callback

JavaScript Callbacks

• A callback is a function passed as an argument to another function
• A callback function can run after another function has finished
  The second argument is a function (usually anonymous) that runs when the action is completed.

Example

function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;
}
 
function myCalculator(num1, num2, myCallback) {
  let sum = num1 + num2;
  myCallback(sum);
}
 
myCalculator(5, 5, myDisplayer);

When you pass a function as an argument, remember not to use parenthesis.
Right: myCalculator(5, 5, myDisplayer);
Wrong: myCalculator(5, 5, myDisplayer());

When to Use a Callback?

Where callbacks really shine are in asynchronous functions, where one function has to wait for another function (like waiting for a file to load).