Javascript--callback (callback function

1. callback function definition

A callback function is a function that is called through a function pointer. If you pass the pointer (address) of the function as an argument to another function, when the pointer is used to invoke the function it points to, we say this is a callback function. The callback function is not called directly by the implementing party of the function, but is invoked by another party when a particular event or condition occurs, and is used to respond to the event or condition.

Therefore, callbacks are essentially a design pattern, and the design principles of jquery (including other frameworks) follow this pattern.

In JavaScript, the callback function is specifically defined as: function A is passed as a parameter (function reference) to another function B, and this function B executes function A. Let's just say that function A is called a callback function. If there is no Name (function expression), it is called an anonymous callback function.

So callback is not necessarily used for asynchrony, and callbacks are often used in scenarios where general synchronization (blocking) is required, such as executing a callback function after certain operations have been performed.

2. When will the callback be executed?

Callback functions, which are typically executed in a synchronous context, may not be executed in an asynchronous context because the event is not triggered or the condition is not satisfied.

3. Application of callback function

• Resource loading: Execute callback after dynamically loading JS file, execute callback after loading IFRAME, Ajax operation Callback, image loading complete execution callback, Ajax and so on.

• DOM events and node. JS events are based on a callback mechanism (the node. JS callback may have multiple layers of callback nesting issues).

• SetTimeout delay Time is 0, this hack is often used, settimeout call function is actually a callback embodiment

• Chained invocation: When chaining calls, it is easy to implement chained calls in the evaluator (setter) method (or in a method that does not have a return value in itself), and the accessor (getter) is relatively bad for chaining calls because you need the accessor to return the data you need instead of the this pointer. If you want to implement a chained method, you can use a callback function to implement

• The function Call of SetTimeout, SetInterval, gets its return value. Since all two functions are asynchronous, that is, their call timing and the program's main process is relatively independent, so there is no way to wait for their return value in the body, they are opened when the program will not stop waiting, otherwise it will lose the meaning of settimeout and setinterval, So with return there is no point, only using callback. The meaning of callback is to notify the agent function of the result of the timer execution to deal with it in time.

4. Metaphor for callback functions

You have to go to the next bedroom to find classmates, found that people are not, how do you do?
Method 1, every few minutes to go to the next bedroom, see people in not
Method 2, please, the man with his bedroom, and see him call you when he comes back.

The former is polling and the latter is a callback.

Is it OK to wait for the classmate to come back directly in the next bedroom?

Yes, just so you can save time to do other things, now you have to waste waiting. Turns the original non-blocking asynchronous call into a blocking synchronous call.

JavaScript callbacks are used in asynchronous invocation scenarios and use callbacks to perform better than polling.

Javascript--callback (callback function