promises and call back

promises and call back are only doing similar job,  but sometimes we have to think whether to have a callback function or promises.

Here we will be discussing about promises and call back function. So let have look on both callback function and promises and how it works.

Call back function

Callback functions are functions that are executed after completing one function.

For example, if we have to do something after getting the result from the server, then we can write a call back function and it will execute once it gets the result from the server.

Let us have a look on the below example:

function first(arg1, callback){
//do some code
//after that call back funtion will be callled
if(type of callback ==function ) 
callback(options);

}

 

promises

Promise is just a way of writing asynchronous code in a more synchronous fashion.
ie, When you call promises, it will return a promise value and the calling code will wait until the promise is full filled, before executing next.

angularjs promises example:

var promise = asyncGreet('Robin Hood');
promise.then(function(greeting) {
alert('Success: ' + greeting);
}, function(reason) {
alert('Failed: ' + reason);
});
 //then(successCallback, errorCallback, notifyCallback)

We can pass three parameters for ".then" function, of which one is on success, 
another one on failure and last one is notifyCallback .notifyCallback can be 
called more than one time to the show progress and it is an optional parameter.

 Some differences between call back promises

  • Call back can be called once or more than once (depends on how it is written).
  • Promises will be called only once
  • Call back can throw errors and exceptions (have to write more call back for errors)
  • Promises have error handling
  • then(fulfilledHandler, errorHandler, progressHandler)
  • No states in call back
  • May have one of 3 states: unfulfilled, fulfilled and failed or only move from unfulfilled to either fulfilled or failed
  • In call back we can call N function in N levels
  • Can write a sequence of N async calls without N levels of indentation

These are some of my view points, if you have any suggestions or addons, you comments are welcome.

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