Javascript Callback Functions
JavaScript callback functions are functions that are passed as arguments to other functions and are executed at a later time. They are commonly used in asynchronous operations, such as handling events or making API requests. Callback functions allow for more flexible and dynamic code by enabling actions to be triggered based on the completion of specific tasks.
3 Key excerpts on "Javascript Callback Functions"
eBook - ePubBeginning ReactJS Foundations Building User Interfaces with ReactJS
An Approachable Guide
- Chris Minnick(Author)
- 2022(Publication Date)
- Wiley(Publisher)
...After the first statement is executed and removed from the call stack, JavaScript sees the setTimeout() function, which creates an event that is only indirectly managed by JavaScript. It hands it off to the browser to execute, and then removes it from the call stack. JavaScript can then move on to the third statement. The Web API, in the meantime, waits for one second (because of the 1,000-millisecond timeout length you passed to it) and then adds your callback function to the browser's callback queue. The event loop (which is in charge of listening for events and registering event listeners in the JavaScript environment) picks up the function from the callback queue and adds it to JavaScript's call stack to be executed. Figure 16-2 shows a diagram of the whole process. Callback functions, which get executed upon completion of an asynchronous task, are how JavaScript programmers can write code that depends on the result of that asynchronous task. If you want to have multiple asynchronous tasks that happen in a particular order, you can put them inside the callback functions from other asynchronous tasks, as shown in Listing 16-1. FIGURE 16-1 : Executing asynchronous JavaScript FIGURE 16-2 : How asynchronous tasks are handled LISTING 16-1 : Callbacks within callbacks function userCheck(username, password, callback){ db.verifyUser(username, password, (error, userInfo) => { if (error) { callback(error) }else{ db.getRoles(username, (error, roles) => { if (error){ callback(error) }else { db.logAccess(username, (error) => { if (error){ callback(error); }else{ callback(null, userInfo, roles); } }) } }) } }) }; In the preceding example, what should happen when the userCheck() function is called (absent any errors) is the following: Verify the user's credentials. Get the user's access permissions. Create a log entry. Nested callbacks can be difficult to read, however, so more intuitive ways to perform tasks in response to asynchronous tasks have been...
eBook - ePub- David Karlins, Doug Sahlin(Authors)
- 2012(Publication Date)
- For Dummies(Publisher)
...These are somewhat flexible and relative terms. JavaScript defines its parts in much more loose and more flexible ways than many other languages. In JavaScript, most things can be considered objects. If you define a new variable (var x;), you’re creating a new object. Shortly, we explore functions in JavaScript, which do things like calculate and place the current date in a document. These functions could also be considered an object as well. The final concept we’ll throw into the hopper is event handlers, which are triggered by actions in a browser. Here are a few examples of event handlers: ♦ onload triggers a JavaScript when a web page opens. ♦ onunload triggers a JavaScript when a web page closes. ♦ onclick triggers a JavaScript when a user clicks an element. We use these event handlers in the examples we are about to explore. Using JavaScript Date Functions Take a minute to quickly revisit the document.write example from earlier in this chapter: <script type=”text/javascript”> document.write(“This is JavaScript”); </script> That code simply prints This is JavaScript on a page...
eBook - ePubConfident Web Design
Master the Fundamentals of Website Creation and Supercharge Your Career
- Kenny Wood(Author)
- 2018(Publication Date)
- Kogan Page(Publisher)
...Now that you understand them, you are so close to being able to claim that you understand all of the core aspects of JavaScript. We have one small section left, before we can put everything we have learned into practice as we go off and create our first dynamic website. Lastly for JavaScript, let’s look at events.EventsIt’s important to remember that JavaScript’s main purpose is to integrate with the browser and your website. JavaScript’s tight integration with the browser is one of the main features of JavaScript. The benefit of using JavaScript with your website is that we are able to write JavaScript code that can ‘react’ to certain ‘events’ that occur on your website.What’s an event? An event is simply something that a browser or a user does. Some examples of events are when a website has finished loading, when a button is clicked, when a user hovers over a div, when a form is submitted and so on. Using JavaScript, we can react to these events by executing code snippets that we define in advance.We can ‘tie’ these actions to the events in our HTML. We can simply assign the action directly to the element we want to react to, like so:<p id=”text”><button onclick=”document.getElementById(‘text’).innerHTML=‘Paul’”>My name is?</button>As you can see, to assign the action to the event of the button being clicked, we add an ‘onclick’ attribute to our HTML tag, followed by the JavaScript action in single or double quotations. The above example will enter the text “Paul” into the div with the id ‘text’ when the button is clicked.As you can see in the above example, we are writing pure JavaScript directly into the attribute’s value, which is fine for small snippets of JavaScript, but what about when we want to write more complex functions? It can quickly become troublesome. As such, it is common in JavaScript to assign functions to the attribute instead of code itself...
