Callback function ----------- callback

One . What is a callback function

according to MDN Description of : The callback function is a function , It will be executed immediately after the execution of another function , A callback function is passed as a parameter to another JavaScript Function of function . This callback function will be executed inside the function passed to .

Two . Why call back

client JavaScrpit Run... In a browser , And the main process of the browser is a single thread time loop . If we try to perform long-running operations in single threaded events , Will stop the process . Technically speaking, this is not good , Because the process stops processing other events while waiting for completion .

for example .alert Statements are treated as browsers JavaScript One of the blocking codes in . If you run alert, Then close alert Before the dialog window , You won't be able to interact in the browser . To prevent blocking long-running operations , We used callback .

3、 ... and . Example

Cool and finish your meal , Can help people bring things , You can also take it without helping others , But rice must be eaten .

Code ：

   function liliang(task) {
      console.log(` Liangliang eats lunch by himself `);
      task()
    }
    liliang(
      function () {
        console.log(` Help Ranran buy steamed stuffed buns `);
      }
    )

Pass a function as a parameter , The reason why the parameter is not written dead , You can entrust more people to do .

Four .JavaScript Is the callback asynchronous ？

JavaScript It is considered a single threaded scripting language , Single thread means Javascrpit Execute one code block at a time . When Javascrpit Busy executing a block , It is impossible to move to the next block .

let me put it another way , We can argue that Javascrpit Code is always blocked by nature . But this kind of blocking makes it impossible for us to write code in some cases , Because in these cases, we can't get immediate results when performing certain tasks .

The tasks I'm talking about include the following ：

1. adopt API Call to get the data .

2. Get some resources from the remote server by sending a network request .

To deal with these situations , You must write asynchronous code , Callback function is a way to deal with these situations . So essentially , The callback function is asynchronous .

5、 ... and . About the callback to hell

When more than one asynchronous function is executed one by one , Or create callback hell .

adopt async Function to solve the problem of callback hell

async yes ES7 New features , Indicates that the current function is asynchronous , It will not block the thread and cause subsequent code to stop running .

Example :

async function Fn() {
      return 'hello world';
    }
    console.log(Fn());

The result returned :

Conclusion :

async Indicates that there is an asynchronous operation in the function

await Indicates that the following expression needs to wait for the result .

6、 ... and . Benefits of callback functions

1. Send a notice at the right time

2. Make the code more flexible

3. Improve the operation efficiency of

Icon :