I think this is so basic so people maybe minus votes on this document, but even so this is so confused me about callback function in JavaScript.
function doSomething(callback){
setTimeout(hello,5000);
callback();
}
function hi(){
console.log("hi");
}
function hello(){
console.log("hello");
}
doSomething(hi);
/* result */
// hi
// (after 5 seconds) hello
I want to use callback function as a handle function's execute order, so I decided use callback pattern. In above code, I think after 5 seconds, the callback function should be executed, but why callback ignore before function and was ran first? Could you tell me a some hint.
Thanks.
In your code callback() was executing after the execution of the line setTimeout() but the callback of setTimeout will trigger after 5000ms, that is the expected behaviour. So if you want callback() to exeute after hello() do:
function doSomething(callback){
setTimeout(function(){
hello();
callback();
},5000);
}
Related
I have some really hard times understanding JS callbacks. From what I understand is that JS callback is a function that takes the argument of another function. So let's say I have a function called myFunction and passes mySecond to it, will it exicute myFunction before mySecond?
function myFunction(){
alert("hello");
}
function mySecond(){
alert("world");}
}
myFunction(mySecond);
if you want myFunction before mySecond. The code is below.
function myFunction(func){
alert("hello");
func();
}
function mySecond(){
alert("world");}
}
myFunction(mySecond);
Hello, if you want myFunction after mySecond. The code is below.
function myFunction(func){
func();
alert("hello");
}
function mySecond(){
alert("world");}
}
myFunction(mySecond);
a JS callback is a function that takes the argument of another function
This "understanding" is confused and has it backwards:
a JS callback is a function provided as an argument to another function, to be called back after the called function has done something.
A classic example or callback function is provided by the syntax of setTimeout:
setTimeout( callback, 1000);
Here callback (a function) is called back (by setTimeout) after a second has elapsed.
I have read a few articles regarding callback function. I understand how they presented like add a + b then give callback function. But I am doing same. I first declared the function then call it again I call the callback function, why it is not working in my case?
function me(callback){
console.log("1")
}
me(function(){
console.log(2)
})
I am expecting console.log 1 then console.log 2. I am getting only console.log 1
you are calling the callback function, it won't trigger automatically, that approach is so you can notify something using that callback function when your function ended something.
function me(callback) {
console.log("1")
// your process ended, lets notify
callback();
}
me(function() {
console.log(2)
})
You have to actually call the callback function inside the function it is passed to as argument:
function me(callback){
console.log(1)
callback();
}
me(function(){
console.log(2);
})
I'm trying to understand how the callback function works inside the setTimeout function. I'm aware the format is: setTimeout(callback, delay) I wrote a little test script to explore this.
test1.js
console.log("Hello")
setTimeout(function () { console.log("Goodbye!") }, 5000)
console.log("Non-blocking")
This works as expected, printing Hello <CRLF> Non-blocking and then 5 seconds later, prints Goodbye!
I then wanted to bring the function outside of the setTimeout like this:
console.log("Hello")
setTimeout(goodbye(), 5000)
console.log("Non-blocking")
function goodbye () {
console.log("Goodbye")
}
but it doesn't work and there isn't a 5 second delay between Non-blocking and Goodbye!, they print straight after each other.
It works if I remove the brackets from the function call in the timeout, like this:
setTimeout(goodbye, 5000)
but this doesn't make sense to me because that's not how you call a function. Futhermore, how would you pass arguments to the function if it looked like this?!
var name = "Adam"
console.log("Hello")
setTimeout(goodbye(name), 5000)
console.log("Non-blocking")
function goodbye (name) {
console.log("Goodbye "+name)
}
My question is really, why doesn't it work when there are parameters in the function, despite the fact the setTimeout is being provided with a valid function with the correct syntax?
By putting the parentheses after your function name, you are effectively calling it, and not passing the function as a callback.
To provide parameters to the function you are calling:
You can pass an anon function. setTimeout(function(){goodbye(name)}, 5000);
Or, you can pass the arguments as a third parameter. setTimeout(goodbye, 5000, name);
Look at this question: How can I pass a parameter to a setTimeout() callback?
No matter where you place it, goodbye(name) executes the function immediately. So you should instead pass the function itself to setTimeout(): setTimeout(goodbye, 5000, name).
When you use it like this:
setTimeout(goodbye(), 5000);
it will first call goodbye to get its return value, then it will call setTimeout using the returned value.
You should call setTimeout with a reference to a callback function, i.e. only specifying the name of the function so that you get its reference instead of calling it:
setTimeout(goodbye, 5000);
To make a function reference when you want to send a parameter to the callback function, you can wrap it in a function expression:
setTimeout(function() { goodbye(name); }, 5000);
You can use parantheses in the call, but then the function should return a function reference to the actual callback function:
setTimeout(createCallback(), 5000);
function createCallback() {
return function() {
console.log("Goodbye");
};
}
So i don't really understand the point of "callback".
Here is an example of callback:
function sayBye(){
alert("Bye!");
}
function saySeeYou(){
alert("See you!");
}
function sayHello(name,myfunc){
alert("Hello");
myfunc;
}
sayHello("Max",saySeeYou());
Whats the point of passing in a function when you can just call the function? like this code does the exact same:
function sayBye(){
alert("Bye!");
}
function saySeeYou(){
alert("See you!");
}
function sayHello(name){
alert("Hello");
saySeeYou();
}
sayHello("Max");
Whats the point of passing in a function when you can just call the function?
Usually, callbacks Javascript are used in Javascript for code that you want to run in the future. The simplest example is setTimeout: if you call the callback now then the code runs immedieately instead of after 500 ms.
//prints with a delay
console.log("Hello");
setTimeout(function(){
console.log("Bye");
}, 500);
//no delay this time
console.log("Hello");
console.log("Bye");
Of course, it would be really neat if we could write something along the lines of
//fake Javascript:
console.log("Hello");
wait(500);
console.log("Bye");
But sadly Javascript doesnt let you do that. Javascript is strictly single-threaded so the only way to code the wait function would be to pause the execution of any scripts in the page for 500 ms, which would "freeze" things in an unresponsive state. Because of this, operations that take a long time to complete, like timeouts or AJAX requests usually use callbacks to signal when they are done instead of blocking execution and then returning when done.
By the way, when passing callbacks you should only pass the function name. If you add the parenthesis you are instead calling the function and passing its return value instead:
//When you write
foo(10, mycallback());
//You are actually doing
var res = mycallback();
foo(10, res);
//which will run things in the wrong order
Your code is not correct as Felix Kling already pointed out. Besides this, passing a function instead of calling one directly allows you to insert different behavior, your code is more decoupled and flexible. Here an example:
function sayBye(){
alert("Bye!");
}
function saySeeYou(){
alert("See you!");
}
function sayHello(name,myfunc){
alert("Hello");
if (myfunc) {
myfunc();
}
}
sayHello("Max",saySeeYou);
// I'm inserting a different behavior. Now instead of displayng "See you!"
// will show "Bye!".
sayHello("Max",sayBye);
You are doing it wrong, you should do like bellow
Don't call the function just pass the function as callback
use this
sayHello("Max",saySeeYou); //here the second parameter is function
instead of
sayHello("Max",saySeeYou());//This will put the result of saySeeYou as second parameter
in say hello call the functiom
function sayHello(name,myfunc){
console.log("Hello");
myfunc();
}
This question already has an answer here:
understanding the concept of javascript callbacks with node.js, especially in loops
(1 answer)
Closed 8 years ago.
I know this has been asked a million times, but I'm really trying to break down async Javascript functions and callbacks and its just not clicking. I'm looking at Max Ogden's Art of Node example which is this:
var fs = require('fs')
var myNumber = undefined
function addOne(callback) {
fs.readFile('number.txt', function doneReading(err, fileContents) {
myNumber = parseInt(fileContents)
myNumber++
callback()
})
}
function logMyNumber() {
console.log(myNumber)
}
addOne(logMyNumber)
Breaking this down, I understand that when addOne is invoked, it first kicks off fs.ReadFile which may take some time to complete.
What I don't get is, won't the code continue to callback() and execute logMyNumber (before myNumber has been added to) anyhow? What's stopping callback() from running before it should, which is the whole point? Or does callback() not happen until doneReading has happened? Are we supposed to assume that doneReading will be invoked when fs.readFile is "done"?
Thank you all for your patience in helping me with this very common question:)
"Are we supposed to assume that doneReading will be invoked when fs.readFile is "done"?"
You dont have to assume it, you can be pretty sure of it.
You can use logging to see how and in what order your code gets executed.
var fs = require('fs')
console.log("starting script");
console.log("message 1");
function addOne(callback) {
fs.readFile('number.txt', function doneReading(err, fileContents) {
console.log("finished loading the file");
console.log("message 2");
callback()
})
}
console.log("message 3");
//logMyNumber will be called after file has read
function logMyNumber() {
console.log("message 4");
}
console.log("message 5");
addOne(logMyNumber)
console.log("message 6");
//______________
A simpler way to understand the asyncronous behavior is to use all familiar timer
console.log("message 1");
var num = 2;
function something() {
console.log("message 2");
}
function somethingElse() {
console.log("message 3");
}
console.log("message 4");
setTimeout(something, 1000);
console.log("message 5");
setTimeout(somethingElse, 500);
//code will run 1 - 4 - 5 - 3- 2 not from top to bottom, and this way its obvious why.
//in file read its the same reason
This is the way the code will flow:
You call to addOne(logMyNumber) will be executed.
addOne will read a file and once the file has been read it will then
execute the code in the "doneReading" function which in turn will call your callback (which is logMyNumber)
See the second argument for fs.readFile? It's a function called doneReading.
fs.readFile will only execute doneReading when it has finished reading the file. When doneReading gets executed, the last line is to call callback(), which is a reference to the logMyNumber function in this case.
fs.readFile will call the given callback, ie. doneReading in your code after the reading has been finished. This is how Node.js generally works with the callbacks: you give the callback which is ran after finishing the asynchronous operation.
// callback is a parameter to addOne
// callback is a function, but functions are just objects in javascript
// so the addOne function just knows that it has one parameter, not that
// callback is a function
function addOne(callback) {
// callback is now captured in the closure for the doneReading function
fs.readFile('number.txt', function doneReading(err, fileContents) {
myNumber = parseInt(fileContents)
myNumber++
// callback is executed here
// But we are inside the doneReading function
// which is itself a callback to the fs.readFile function
// therefore, it does not get executed until the file has finished reading
callback()
})
}
// similarly, logMyNumber has not been called, it has just been defined
// as a function (object)...
function logMyNumber() {
console.log(myNumber)
}
// ...and passing logMyNumber to addOne here does not execute it
addOne(logMyNumber)
Does that clear it up?