JS Tutorials
JS Introduction
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JS Variables
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JS Data Types
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JS Functions
Function Definition
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Function ClosuresJS Classes
Class Intro
Class Inheritance
Class Static
JS Async
JS Callbacks
JS Aysnchronous
JS Promises
JS Sync/Await
Previous
Asynchronous JavaScript
The examples used in the previous chapter, was very simplified.
The purpose of the examples was to demonstrate the syntax of callback functions:
Example
function myDisplayer(some) {
document.getElementById("demo").innerHTML
= some;
}
function myCalculator(num1, num2, myCallback) {
let sum = num1 + num2;
myCallback(sum);
}
myCalculator(5, 5, myDisplayer);
In the example above, myDisplayer is the name of a function.
It is passed to myCalculator() as an argument.
In the real world, callbacks are most often used with asynchronous functions. A typical example is JavaScript setTimeout().
Waiting for a Timeout
When using the JavaScript function setTimeout(), you can specify a callback function to be executed on time-out:
function myFunction() {
document.getElementById("demo").innerHTML = "I love You !!";
}
In the example above, myFunction is used as a callback.
The function (the function name) is passed to setTimeout() as an argument.
3000 is the number of milliseconds before time-out, so myFunction will be called after 3 seconds.
When you pass a function as an argument, remember not to use parenthesis.
Right: setTimeout(myFunction, 3000);
Wrong: setTimeout(myFunction(), 3000);
Instead of passing the name of a function as an argument to another function, you can always pass a whole function instead:
setTimeout(function() { myFunction("I love You !!!"); }, 3000);
function myFunction(value) {
document.getElementById("demo").innerHTML = value;
}
In the example above, function(){ myFunction("I love You !!!"); } is used as a callback. It is a complete function. The complete function is passed to setTimeout() as an argument.
3000 is the number of milliseconds before time-out, so myFunction() will be called after 3 seconds.