¿有人可以向我解释 JavaScript 中的回调吗？ - 初学者

Question

i im a beginner in this world and i feel like a im a little bit loset in thist.

i buy a course in udemy and this is the example:

function add( a, b, cb) {
    const r = a + b 
    cb(r)
}

function callback(result) {
    console.log('This is the result', result);
}

add(2, 3, callback)

This is the part i dont understand: cb(r) - ¿Why the "cb" are in that part of my code (add(a,b, cb )) and repeat later? i undestan we save the result of const r in Cb(r) but i dont know why is in both parts

¿Why when i run this in my terminal it came callback function firts and add function?

if someone can i explain to me what are happen here step by step i goint to appreciate it

i just whant to know how this works because its sound like is important on this path, it runs but i dont know why

Answer 1

The order in which you wrote function add and function callback doesn't matter. They're just definitions, ie recipes on how to do something. But unless they're called, nothing is actually done by just writing functions.

By writing function add( a, b, cb) , you just say " I am writing a function named add, which is expected to receive 3 parameters as an input, and I name them a, b and cb. " But you don't know yet what their value is. You just name them this way in order to be able to do things with them. When you call cb(r) you have basically no idea what cb is, you just know it's a function to be completed later.

Basically your code is not going to be executed in the order in which you wrote it.

Javascript starts doing things when it reads add(2, 3, callback) . At this point it understands that it needs to execute function add , to which you've given three parameters, two of which are number and the third one is a function. But it doesn't execute the function callback yet, it just knows its name. For now it's just going to search into your code where the function add is and execute this one first, with a=2, b=3, and cb=callback .

Once it found it, jumps to the beginning of the function add , creates a new constant r and puts a+b in it. When it sees Cb(r) , it takes your third parameter that you've submitted to it, in this case the function callback , and calls it with parameter r . So again, it jumps to the beginning of function callback and executes it with parameter r .

In this example, there is no need to use a callback function, that's just a basic exemple to illustrate the concept of callbacks. But as said in the comments of your post, you may want to write a second callback function, such as:

function anotherCallback(result) {
    console.log('This is ANOTHER result', result);
}

And then you could call

add(2, 3, callback)
add(4, 5, anotherCallback)

without having to re-write the function add.

I've tried to make it as detailed as possible, I hope this is clear but if not don't hesitate to ask further questions.