Callback Function Javascript

The consumer of a callback-based API writes a function that is passed into the API. The provider of the API (called the caller) takes the function and calls back (or executes) the function at some point inside 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.

There are two ways in which the callback may be called: synchronous and asynchronous. Synchronous callbacks are called immediately after the invocation of the outer function, with no intervening asynchronous tasks, while asynchronous callbacks are called at some point later, after an asynchronous operation has completed.

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If doSomething calls the callback synchronously, then the last statement would log 2 because value = 2 is synchronously executed; otherwise, if the callback is asynchronous, the last statement would log 1 because value = 2 is only executed after the console.log statement.

Examples of synchronous callbacks include the callbacks passed to Array.prototype.map(), Array.prototype.forEach(), etc. Examples of asynchronous callbacks include the callbacks passed to setTimeout() and Promise.prototype.then().

Callbacks make sure that a function is not going to run before a task is completed but will run right after the task has completed. It helps us develop asynchronous JavaScript code and keeps us safe from problems and errors.

I was reading about callback functions here (also in an online course which I am participating) and now I am stuck. The reason is that I cannot understand Why do I need to use callback functions if I can simply call them. Exemples below:

The filter function on arrays is an example of #1: It calls the callback for each entry in the array, using the return value of the callback to decide whether the entry should be in the new, filtered array it returns:

The Promise methods then, catch, and finally are examples of #2. Let's assume we have a startSomethingAsynchronous function that returns a promise. Here's how the fulfillment and rejection handlers (callbacks) might be set up:

We need callback functions because many JavaScript actions are asynchronous, which means they don't really stop the program (or a function) from running until they're completed, as you're probably used to. Instead, it will execute in the background while the rest of the code runs.

A callback's primary purpose is to execute code in response to an event. These events might be user-initiated, such as mouse clicks or typing. With a callback, you may instruct your application to "execute this code every time the user clicks a key on the keyboard."

In the above code, we add addEventListener as a function and we are passing another function callback as an argument. And when a click event is triggered the addEventListener registers the callback function.

If you wanted to download data from a server (which may take long durations of time), it would be wasteful for your program or function to just freeze while you waited for the data to be fetched. Instead, it's common practice to execute the fetching operation in the background.

This implies that if you have two functions in a row, one of which is asynchronous, function B will be performed while the other is still running. If function B is dependent on the data that function A is retrieving, you will run into issues.

The asynchronous functions return a promise, when the asynchronous function encounters the term await (notice that fetch() delivers a promise), it stops its execution until the promise is resolved. To read more about promises click here.

An asynchronous callback function differs from an asynchronous function. The higher-order function executes the asynchronous callback function in a non-blocking way. However, while waiting for promises (await promise>) to resolve, the asynchronous function stops its execution.>

We can write asynchronous functions as if they are synchronous (executed sequentially) with the use of await as it stops the execution until the promise is resolved (function execution is successful).

Callback functions are needed because many JavaScript actions are asynchronous. Instead, it executes in the background while the rest of the code runs. A callback's purpose is to execute code in response to an event. These events can be like mouse clicks; with callback, we can add text at the end of each statement like"execute this code every time the user clicks a key on the keyboard."

Callbacks ensure that the function will not run before the task is completed but will run right after the task is completed. It develops asynchronous JavaScript code and keeps them safe from errors. In JavaScript, the way to create a callback function is to pass it as a parameter to another function and then call it back after the task is completed.

Here, the greet() function is called after 2000 milliseconds (2 seconds). During this wait, the sayName('John'); is executed. That is why Hello John is printed before Hello world.

In the above example, the second function does not wait for the first function to be complete. However, if you want to wait for the result of the previous function call before the next statement is executed, you can use a callback function. For example,

A callback is a function that is passed as an argument to another function, and is called after the main function has finished its execution. The main function is called with a callback function as its argument, and when the main function is finished, it calls the callback function to provide a result. Callbacks allow you to handle the results of an asynchronous operation in a non-blocking manner, which means that the program can continue to run while the operation is being executed.

Callbacks allow you to continue executing code while the operation is being executed in the background. Once the operation has completed, the callback function is called with the result of the operation. This way, you can ensure that the program remains responsive and the user experience is not impacted.

Asynchronous programming: Callbacks are used to handle the results of asynchronous operations, which means that the operation does not block the execution of the rest of the program. Instead, the program continues to run and the callback function is executed when the operation is complete.

Higher-order functions: A higher-order function is a function that takes one or more functions as arguments, or returns a function as a result. The mainFunction in the examples above is a higher-order function because it takes a callback function as an argument.

Anonymous functions: Anonymous functions are functions that are not named and are often used as callbacks. The function passed to setTimeout in the first code example is an anonymous function.

Closure: A closure is a function that has access to variables in its outer scope, even after the outer function has returned. This allows the callback function to access variables and information from the main function, even after the main function has completed its execution.

Handling form submissions: When a user submits a form, it takes time to process the data and send it to the server. If the form submission was executed synchronously, the user would have to wait for the data to be processed and sent before the form can be submitted. With callbacks, you can handle the form submission asynchronously, which means that the user can continue to interact with the form while the data is being processed and sent in the background

I'm a little confused, not about what callbacks are used for but because I see callbacks used in scenarios often where it seems like passing a function as an argument to another function isn't necessary because you can just invoke the function you want invoked after the code in the current function is finished executing. For example:

I used a callback there but I could've just called the function myFunction after the code executed without passing myFunction as an argument. I understand a framework or library doesn't know the name of the function you'll want called after code is finished executing so passing the function you want called later as an argument seems to make sense then but when you know the name of the function you want invoked after whatever code is executed in a function, why not just invoke it just as I did in the first example I gave? I would appreciate your help. Thanks e24fc04721