Callback
2. Callback place vital role to execute required code after async code completes.
3. Callback is a function that is passed to another function as a parameter, and Callback function gets called inside the another function
different ways to call Callback functions
$("#btnhome").click(function() {
alert("Home Clicked");
});
Here, click method expecting function as parameter.
So, anonymous function passing as a parameter
2. Passing named function as callback function
alert("Home Clicked");
}
$("#btnhome").click(displayalert);
Here, click method expecting function as parameter.
So, named function passing as a parameter
var displaymessage=function()
{
alert("message display");
}
var showAlert=function(callback)
{
console.log("showing alert message");
callback();
}
// calling function
showAlert(displaymessage);
3. Callback function with parameters
var add=function(a,b)
{
return a+b;
}
var calculate=(num1,num2,callback)
{
Console.log("adding numbers");
var result=callback(num1,num2);
Console.log(result);
}
// calling function
calculate(2,5,add);
Promise
2. It avoids nesting Callback and multiple try/catch
3. It represent asynchronous operations as a sequence
4. It is a different syntax for achieving the same effect as callbacks. The advantage is increased readability
5. A promise represents the result of an asynchronous operation. It is a placeholder into which the successful result value or reason for failure will materialize
6. A promise contain callback function two arguments, resolve and reject , which are both functions. All your asynchronous code goes inside that callback. If everything is successful, the promise is fulfilled by calling resolve() . In case of an error, reject() is called with an Error object. This indicates that the promise is rejected
Example:
var p= new Promise(function(resolve, reject) {
if (true) {
resolve("successfull");
}
else {
reject(Error("failed"));
}
});
// calling Promise
p.then(function(response) {
console.log("Success!", response);
}).catch(function(error) {
console.log("Failed!", error);
})
Promise Chaining : It is used to call multiple async operations one after another
asyncThing1().then(function() {
return asyncThing2();
}).then(function() {
return asyncThing3();
}).catch(function(err) {
console.log('error occured',err);
})
Callback with Nesting
api(function(result){
api2(function(result2){
api3(function(result3){
// do work
});
});
});
Promise with Nesting
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
});
Happy Coding :)
