Computer Science
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.
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3 Key excerpts on "Javascript Callback Functions"
No longer available |Learn moreAdvanced JavaScript
Speed up web development with the powerful features and benefits of JavaScript
- Zachary Shute(Author)
- 2019(Publication Date)
- Packt Publishing(Publisher)
Process the function and add the necessary event and handler info to the heap. Remove the event and handler in the next step.- Push to the event queue if an event completes.
- Pull from the event queue and call the handler function.
- Repeat this for the rest of the steps (first 10 only).
Code https://bit.ly/2R5YGPA OutcomeFigure 2.4: Scope outputsFigure 2.5: Scope outputsFigure 2.6: Scope outputs You have successfully demonstrated a simplified form of how the Event Loop handles the stack.Callbacks
Callbacks are the most basic form of JavaScript asynchronous programming. In the simplest terms, a callback is a function that gets called after another function completes. Callbacks are used to handle the response of an asynchronous function call.In JavaScript, functions are treated like objects. They can be passed around as arguments, returned by functions, and saved into variables. A callback is a function object that is passed as an argument into a higher order function. A higher order function is simply a mathematics and computer science term for a function that takes one or more functions as arguments (callbacks) or returns a function. In JavaScript, a higher order function will take a callback as a parameter. Once the higher order finishes doing some form of work, such as an HTTP request or database call, it calls the callback function with the error or return values.As mentioned in the Event Loop section in Asynchronous Programming , JavaScript is an event driven language. Since JavaScript is single threaded, any long-running operations are blocking. JavaScript handles this blocking effect by using events. When an operation completes and event fires, the event has an attached handler function that gets called to handle the result. These functions are callbacks . Callbacks are the key that allow JavaScript events to perform work when handling asynchronous events.Building Callbacks
Callbacks in JavaScript follow a simple unofficial convention. A callback function should take in at least two arguments: error and result
No longer available |Learn moreThe JavaScript Workshop
A New, Interactive Approach to Learning JavaScript
- Joseph Labrecque, Jahred Love, Daniel Rosenbaum, Nick Turner, Gaurav Mehla, Alonzo L. Hosford, Florian Sloot, Philip Kirkbride(Authors)
- 2019(Publication Date)
- Packt Publishing(Publisher)
Linux Terminal would simply read the contents of a directory on the hard disk and display the details of those files and directories within the Terminal window. The Linux operating system is built on the premise of such very small and simple applications working together to create a much larger ecosystem. The converse of this may be modern multiplayer video games, which typically react to user interaction and receive streamed data from remote locations. The former of these concepts can be considered much like a function: input enters from the top and is output somewhere within the body, typically, the end of the function.JavaScript applications can facilitate both ends of this spectrum, and indeed, anything in between. Modern browsers are now fully capable of providing the foundations for immense and processor-intensive 3D multiplayer games, responding to data from numerous sources, but JavaScript is also frequently used for the simplest of tasks, such as formatting a string or rounding a number .At the core of all of these applications are events . Events, conceptually speaking, are triggers that execute code. This might, for example, be the ready state when a page has finished loading or a mouse event when the user clicks an element within the page. Typically, without events, functions won't know when to execute and, therefore, nothing can happen.Throughout this chapter, we will examine the options JavaScript provides for listening to and handling different types of events within the browser environment.Event Types
An event is simply a notification or a "triggered " alert within the JavaScript runtime. These notifications can represent practically anything but are a means to invoke one or more of your own functions when such an event occurs.When a web page loads, the browser will typically display content as soon as it is available. This means some content will be presented to the user before the entire page has finished downloading. The browser does this to prevent long-loading assets from withholding other content from being available to the user.Now, imagine you want to invoke a function immediately within a web page to rotate an image. JavaScript code embedded into a web page is able to run immediately once it has been parsed by the JavaScript engine, which could possibly be before the image in question is available. To overcome this conundrum, JavaScript provides an onload
eBook - ePubNode.js Design Patterns
Design and implement production-grade Node.js applications using proven patterns and techniques, 3rd Edition
- Mario Casciaro, Luciano Mammino(Authors)
- 2020(Publication Date)
- Packt Publishing(Publisher)
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Callbacks and Events
In synchronous programming, we conceptualize code as a series of consecutive computing steps that solve a specific problem. Every operation is blocking, which means that only when an operation is completed, it is possible to execute the next one. This approach makes the code very easy to read, understand, and debug.On the other side, in asynchronous programming, some operations, such as reading from a file or performing a network request, are launched and then executed "in the background." When we invoke an asynchronous operation, the instruction that follows is executed immediately, even if the previous asynchronous operation has not finished yet. In this scenario, we need a way to get notified when an asynchronous operation completes, and then continue the execution flow using the results from the operation. The most basic mechanism to get notified about the completion of an asynchronous operation in Node.js is the callback , which is nothing more than a function invoked by the runtime with the result of an asynchronous operation.The callback is the most basic building block on which all other asynchronous mechanisms are based. In fact, without callbacks, we wouldn't have promises, and therefore not even async/await; we also wouldn't have streams or events. This is why it's important to know how callbacks work.In this chapter, you will learn more about the Node.js Callback pattern and understand what it means, in practice, to write asynchronous code. We will make our way through conventions, patterns, and pitfalls, and by the end of this chapter, you will have mastered the basics of the Callback pattern.You will also learn about the Observer pattern, which can be considered a close relative of the Callback pattern. The Observer pattern—embodied by the EventEmitter
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