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ammarkhan
Front End Web Development Techdegree Student 21,661 PointsConstructor args and promises
I am new to constructor, as I cannot understand when Andrew passed Http:http into the constructor like
constructor(http:Http){
}
and also I am not familiar with promises, does Treehouse have a course which teaches javascript promises?
1 Answer
Daniel H.J. Chen
12,888 PointsWhat this line does is that when the component is initiated, the constructor function creates a private variable "http" which is of type "Http", the service class just imported from '@angular/http'. Then, "http" and its methods like "get" and "post" can be accessed in the component.
Promise is like an event listener for a functional call. You set a promise so that when the functional call finishes executing, the promise evaluates whether the functional call is successful, then send the processing to either the success callback (fulfilled) or failure callback (rejected). The Angular Http calls are Observables, which you convert to Promises and pass the response to appropriate callbacks. You can look up Promises in the following link: https://developers.google.com/web/fundamentals/getting-started/primers/promises
ammarkhan
Front End Web Development Techdegree Student 21,661 Pointsammarkhan
Front End Web Development Techdegree Student 21,661 PointsCool, this bring me to ask what are Observables?
Daniel H.J. Chen
12,888 PointsDaniel H.J. Chen
12,888 PointsAn Observable is very similar to a Promise as they are both abstract layer for asynchronous calls, except that you have more flexibility. As of ES6, you cannot cancel a Promise - it will always result in success or failure and call the respective callbacks. For Observables, you can cancel via subscriptions and call retries easily. Check more at https://stackoverflow.com/questions/37364973/angular-promise-vs-observable