Callback Functions

Overview

Functions passed as arguments to other functions, which is then invoked inside the outer function to complete some kind of routine or action

• Important for asynchronous programming

• The consumer of a callback-based API writes a function passed into the API
• The provider / caller of the API takes the function and “calls back” or executes the function at some point in the caller’s body
  • The caller is responsible for passing the right parameters into the callback function
  • The caller may also expect a particular return value from the callback function, which is used to instruct further behavior of the caller
• 2 ways the callback may be called
  • synchronous
    • called immediately after the invocation of the outer function w/ no intervening asynchronous tasks
  • asynchronous
    • asynchronous callbacks are called at some point later, after an asynchronous operation has completed
    • setTimeout and setInterval

Example 1

/// ======= 1st scenario =======
var callback = function()
{
	console.log("Done!");
}
 
// waits 5 seconds and prints "Done!" when the 5 seconds are up
setTimeout(callback, 5000);
 
/// ======= 2nd scenario =======
setTimeout(function()
{
	console.log("Done!");
}, 5000)

• just functions that get executed when something happens (like a click) or when some operation finishes (like a timer)

Example 2

// ===================================== another example
 
function useCallback(fn) {
    fn("Hello");        // This runs first
    fn("How are you?"); // This runs second
    fn("Goodbye!");     // This runs third
}
 
function printMsg(message) { // the callback
    console.log(message);
}
 
useCallback(printMsg);