I want to know how the "spyOn" function works internally. I read that the 'spyOn' function internally replaces the implementation of the function being spied on. Does it keep the old functionality?
As an example, suppose I wanted to spy on an object that sends data to a server.
describe("A spy", function() {
var object;
spyOn(object, 'sendDataToServer');
object.sendDataToServer('Some Message');
});
In this case, does the message still get sent to the server or does the spy mock it?
The message does not get sent to the server. The way you defined the spy, it will replace the sendDataToServer function whenever it is called in the context of your test case.
You can specify a more elaborate spy, for example when you want to call another function instead:
let mySpy = spyOn(object, 'sendDataToServer').and.callFake((message: string) => {
console.log('I have been called with ' + message);
});
object.sendDataToServer('Some Message'); // -> will call the function defined above and log the message passed
Or if you want to call the actual function:
let mySpy = spyOn(object, 'sendDataToServer').and.callThrough();
object.sendDataToServer('Some Message'); // -> will call the actual function on object
I've been developing JavaScript for a few years and I don't understand the fuss about promises at all.
It seems like all I do is change:
api(function(result){
api2(function(result2){
api3(function(result3){
// do work
});
});
});
Which I could use a library like async for anyway, with something like:
api().then(function(result){
api2().then(function(result2){
api3().then(function(result3){
// do work
});
});
});
Which is more code and less readable. I didn't gain anything here, it's not suddenly magically 'flat' either. Not to mention having to convert things to promises.
So, what's the big fuss about promises here?
Promises are not callbacks. A promise represents the future result of an asynchronous operation. Of course, writing them the way you do, you get little benefit. But if you write them the way they are meant to be used, you can write asynchronous code in a way that resembles synchronous code and is much more easy to follow:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
});
Certainly, not much less code, but much more readable.
But this is not the end. Let's discover the true benefits: What if you wanted to check for any error in any of the steps? It would be hell to do it with callbacks, but with promises, is a piece of cake:
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
});
Pretty much the same as a try { ... } catch block.
Even better:
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
}).then(function() {
//do something whether there was an error or not
//like hiding an spinner if you were performing an AJAX request.
});
And even better: What if those 3 calls to api, api2, api3 could run simultaneously (e.g. if they were AJAX calls) but you needed to wait for the three? Without promises, you should have to create some sort of counter. With promises, using the ES6 notation, is another piece of cake and pretty neat:
Promise.all([api(), api2(), api3()]).then(function(result) {
//do work. result is an array contains the values of the three fulfilled promises.
}).catch(function(error) {
//handle the error. At least one of the promises rejected.
});
Hope you see Promises in a new light now.
Yes, Promises are asynchronous callbacks. They can't do anything that callbacks can't do, and you face the same problems with asynchrony as with plain callbacks.
However, Promises are more than just callbacks. They are a very mighty abstraction, allow cleaner and better, functional code with less error-prone boilerplate.
So what's the main idea?
Promises are objects representing the result of a single (asynchronous) computation. They resolve to that result only once. There's a few things what this means:
Promises implement an observer pattern:
You don't need to know the callbacks that will use the value before the task completes.
Instead of expecting callbacks as arguments to your functions, you can easily return a Promise object
The promise will store the value, and you can transparently add a callback whenever you want. It will be called when the result is available. "Transparency" implies that when you have a promise and add a callback to it, it doesn't make a difference to your code whether the result has arrived yet - the API and contracts are the same, simplifying caching/memoisation a lot.
You can add multiple callbacks easily
Promises are chainable (monadic, if you want):
If you need to transform the value that a promise represents, you map a transform function over the promise and get back a new promise that represents the transformed result. You cannot synchronously get the value to use it somehow, but you can easily lift the transformation in the promise context. No boilerplate callbacks.
If you want to chain two asynchronous tasks, you can use the .then() method. It will take a callback to be called with the first result, and returns a promise for the result of the promise that the callback returns.
Sounds complicated? Time for a code example.
var p1 = api1(); // returning a promise
var p3 = p1.then(function(api1Result) {
var p2 = api2(); // returning a promise
return p2; // The result of p2 …
}); // … becomes the result of p3
// So it does not make a difference whether you write
api1().then(function(api1Result) {
return api2().then(console.log)
})
// or the flattened version
api1().then(function(api1Result) {
return api2();
}).then(console.log)
Flattening does not come magically, but you can easily do it. For your heavily nested example, the (near) equivalent would be
api1().then(api2).then(api3).then(/* do-work-callback */);
If seeing the code of these methods helps understanding, here's a most basic promise lib in a few lines.
What's the big fuss about promises?
The Promise abstraction allows much better composability of functions. For example, next to then for chaining, the all function creates a promise for the combined result of multiple parallel-waiting promises.
Last but not least Promises come with integrated error handling. The result of the computation might be that either the promise is fulfilled with a value, or it is rejected with a reason. All the composition functions handle this automatically and propagate errors in promise chains, so that you don't need to care about it explicitly everywhere - in contrast to a plain-callback implementation. In the end, you can add a dedicated error callback for all occurred exceptions.
Not to mention having to convert things to promises.
That's quite trivial actually with good promise libraries, see How do I convert an existing callback API to promises?
In addition to the already established answers, with ES6 arrow functions Promises turn from a modestly shining small blue dwarf straight into a red giant. That is about to collapse into a supernova:
api().then(result => api2()).then(result2 => api3()).then(result3 => console.log(result3))
As oligofren pointed out, without arguments between api calls you don't need the anonymous wrapper functions at all:
api().then(api2).then(api3).then(r3 => console.log(r3))
And finally, if you want to reach a supermassive black hole level, Promises can be awaited:
async function callApis() {
let api1Result = await api();
let api2Result = await api2(api1Result);
let api3Result = await api3(api2Result);
return api3Result;
}
In addition to the awesome answers above, 2 more points may be added:
1. Semantic difference:
Promises may be already resolved upon creation. This means they guarantee conditions rather than events. If they are resolved already, the resolved function passed to it is still called.
Conversely, callbacks handle events. So, if the event you are interested in has happened before the callback has been registered, the callback is not called.
2. Inversion of control
Callbacks involve inversion of control. When you register a callback function with any API, the Javascript runtime stores the callback function and calls it from the event loop once it is ready to be run.
Refer The Javascript Event loop for an explanation.
With Promises, control resides with the calling program. The .then() method may be called at any time if we store the promise object.
In addition to the other answers, the ES2015 syntax blends seamlessly with promises, reducing even more boilerplate code:
// Sequentially:
api1()
.then(r1 => api2(r1))
.then(r2 => api3(r2))
.then(r3 => {
// Done
});
// Parallel:
Promise.all([
api1(),
api2(),
api3()
]).then(([r1, r2, r3]) => {
// Done
});
Promises are not callbacks, both are programming idioms that facilitate async programming. Using an async/await-style of programming using coroutines or generators that return promises could be considered a 3rd such idiom. A comparison of these idioms across different programming languages (including Javascript) is here: https://github.com/KjellSchubert/promise-future-task
No, Not at all.
Callbacks are simply Functions In JavaScript which are to be called and then executed after the execution of another function has finished. So how it happens?
Actually, In JavaScript, functions are itself considered as objects and hence as all other objects, even functions can be sent as arguments to other functions. The most common and generic use case one can think of is setTimeout() function in JavaScript.
Promises are nothing but a much more improvised approach of handling and structuring asynchronous code in comparison to doing the same with callbacks.
The Promise receives two Callbacks in constructor function: resolve and reject. These callbacks inside promises provide us with fine-grained control over error handling and success cases. The resolve callback is used when the execution of promise performed successfully and the reject callback is used to handle the error cases.
No promises are just wrapper on callbacks
example
You can use javascript native promises with node js
my cloud 9 code link : https://ide.c9.io/adx2803/native-promises-in-node
/**
* Created by dixit-lab on 20/6/16.
*/
var express = require('express');
var request = require('request'); //Simplified HTTP request client.
var app = express();
function promisify(url) {
return new Promise(function (resolve, reject) {
request.get(url, function (error, response, body) {
if (!error && response.statusCode == 200) {
resolve(body);
}
else {
reject(error);
}
})
});
}
//get all the albums of a user who have posted post 100
app.get('/listAlbums', function (req, res) {
//get the post with post id 100
promisify('http://jsonplaceholder.typicode.com/posts/100').then(function (result) {
var obj = JSON.parse(result);
return promisify('http://jsonplaceholder.typicode.com/users/' + obj.userId + '/albums')
})
.catch(function (e) {
console.log(e);
})
.then(function (result) {
res.end(result);
}
)
})
var server = app.listen(8081, function () {
var host = server.address().address
var port = server.address().port
console.log("Example app listening at http://%s:%s", host, port)
})
//run webservice on browser : http://localhost:8081/listAlbums
JavaScript Promises actually use callback functions to determine what to do after a Promise has been resolved or rejected, therefore both are not fundamentally different. The main idea behind Promises is to take callbacks - especially nested callbacks where you want to perform a sort of actions, but it would be more readable.
Promises overview:
In JS we can wrap asynchronous operations (e.g database calls, AJAX calls) in promises. Usually we want to run some additional logic on the retrieved data. JS promises have handler functions which process the result of the asynchronous operations. The handler functions can even have other asynchronous operations within them which could rely on the value of the previous asynchronous operations.
A promise always has of the 3 following states:
pending: starting state of every promise, neither fulfilled nor rejected.
fulfilled: The operation completed successfully.
rejected: The operation failed.
A pending promise can be resolved/fullfilled or rejected with a value. Then the following handler methods which take callbacks as arguments are called:
Promise.prototype.then() : When the promise is resolved the callback argument of this function will be called.
Promise.prototype.catch() : When the promise is rejected the callback argument of this function will be called.
Although the above methods skill get callback arguments they are far superior than using
only callbacks here is an example that will clarify a lot:
Example
function createProm(resolveVal, rejectVal) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (Math.random() > 0.5) {
console.log("Resolved");
resolve(resolveVal);
} else {
console.log("Rejected");
reject(rejectVal);
}
}, 1000);
});
}
createProm(1, 2)
.then((resVal) => {
console.log(resVal);
return resVal + 1;
})
.then((resVal) => {
console.log(resVal);
return resVal + 2;
})
.catch((rejectVal) => {
console.log(rejectVal);
return rejectVal + 1;
})
.then((resVal) => {
console.log(resVal);
})
.finally(() => {
console.log("Promise done");
});
The createProm function creates a promises which is resolved or rejected based on a random Nr after 1 second
If the promise is resolved the first then method is called and the resolved value is passed in as an argument of the callback
If the promise is rejected the first catch method is called and the rejected value is passed in as an argument
The catch and then methods return promises that's why we can chain them. They wrap any returned value in Promise.resolve and any thrown value (using the throw keyword) in Promise.reject. So any value returned is transformed into a promise and on this promise we can again call a handler function.
Promise chains give us more fine tuned control and better overview than nested callbacks. For example the catch method handles all the errors which have occurred before the catch handler.
Promises allows programmers to write simpler and far more readable code than by using callbacks.
In a program, there are steps want to do in series.
function f() {
step_a();
step_b();
step_c();
...
}
There's usually information carried between each step.
function f() {
const a = step_a( );
const b = step_b( a );
const c = step_c( b );
...
}
Some of these steps can take a (relatively) long time, so sometimes you want to do them in parallel with other things. One way to do that is using threads. Another is asynchronous programming. (Both approaches has pros and cons, which won't be discussed here.) Here, we're talking about asynchronous programming.
The simple way to achieve the above when using asynchronous programming would be to provide a callback which is called once a step is complete.
// step_* calls the provided function with the returned value once complete.
function f() {
step_a(
function( a )
step_b(
function( b )
step_c(
...
)
},
)
},
)
}
That's quite hard to read. Promises offer a way to flatten the code.
// step_* returns a promise.
function f() {
step_a()
.then( step_b )
.then( step_c )
...
}
The object returned is called a promise because it represents the future result (i.e. promised result) of the function (which could be a value or an exception).
As much as promises help, it's still a bit complicated to use promises. This is where async and await come in. In a function declared as async, await can be used in lieu of then.
// step_* returns a promise.
async function f()
const a = await step_a( );
const b = await step_b( a );
const c = await step_c( b );
...
}
This is undeniably much much more readable than using callbacks.
Gist:
I spy on get method of my rest service:
spyOn(restService, 'get').and.callFake(function () {
return deferred.promise;
});
The method I am trying to test is myService.getFormData() that returns a chained promise:
function getFormData() {
var getPromise = this.restService.get(endPoint, params, true);
var processedDataPromise = then(successHandle, failHandler);
return processedDataPromise;
}
Back to Jasmine spec, I invoke getFormData function and make assertions:
var processedDataPromise = myService.getFormData();
processedDataPromise.then(function(data) {
expect(data).not.toBeNull();
});
deferred.resolve(testFormData);
$rootScope.$digest();
The Problem:
The above derivative promise (processedDataPromise) does indeed get resolved. However the 'data' passed to it is undefined. Is it anything to do with $digest cycles not doing its job in Jasmine?
Why does Jasmine not pass any data to the above derived promise.?
Further Note: The processedDataPromise is a completely new promise than the get returned promise.
It is a promise for processedData which as we can see is returned by successHandle (Definition not shown) once its parent getPromise gets resolved.
In UI everything works like a Charm.
Sorry for posting the question. The derivative promise indeed got the resolved data I was referring to. The problem was that I was incorrectly accessing the JSON data inside of successHandle.
As a result the 'successHandle' returned null and the the processedDataPromise got back undefined response.
Stupid mistake, very hard to find, but the best part is the learning and understanding of JS Promises.
I'm trying to create a amd module that runs a d3 request (d3.json() in this case) and returns the data from the request. I can't seem to figure out how to make the module wait for the request to finish before it returns the data. As a result I keep getting undefined in my main program when I try to access the data.
define(['app/args'], function(args){
d3.json("resources/waterData.php?stn=" + args.stationID, function (error, data) {
var dataToReturn = {};
//Do some stuff with data
return dataToReturn;
});
});
That is the basic structure of what I'm trying to do. I think the main issue is that the 2nd argument in d3.json is a callback for when the data is loaded, so when I try to return the data, it isn't getting outside the module. I haven't been able to figure out how to get the data from the callback to return it outside the module.
The real issue is that the d3.json function is asynchronous, so you can't just return the processed data from the outside function directly. One way you can work around this is by returning a promise rather than the data itself. You can use the d3.json callback to resolve the promise, and then other modules which depend on the data can register their own callbacks which will run once that promise has been resolved.
For example, if you use jQuery's $.Deferred() you can do the following:
define(['app/args', 'jquery'], function(args, $){
// CREATE DEFERRED OBJECT
var deferred = $.Deferred();
d3.json("resources/waterData.php?stn=" + args.stationID, function (error, data) {
var dataToReturn = {};
//Do some stuff with data
// RESOLVE NOW THAT THE DATA IS READY
deferred.resolve(dataToReturn);
});
// RETURN THE PROMISE
return deferred.promise();
});
Then when you want to use the data, you can require the above module, and register a listener that will fire once the data is ready:
require(['nameOfYourModuleAbove'], function(deferred) {
deferred.done(function(data) {
// DO SOMETHING WITH YOUR DATA
});
})
Using Knockout 2.0 and MVC3 Razor forms, I am not able to make a dependent observable work when I introduce an ajax method. I have set up a set of observables that are a part of a calculation, and when I return the product of those observables, I am able to set a SPAN tag with the correct result. However, when I try to use the ajax method to handle those observables and return a result, I get unpredictable behavior. First, it appears the ajax POST does not pick up one of the observables when the INPUT fields are updated (var b POSTs to the action method as 0, but then is eventually updated), and then it seems that I am not able to set the result even when it evaluates correctly. It appears there is timing issue with either the observables or the ajax call. Although simply keeping performing the calculation in javascript works fine, my intent is to call ajax methods for more complicated logic. I have removed the call to ko.applybindings from doc.ready(), and also moved the SCRIPT methods to the bottom of the page- this was the only way I found to make this partly functional. My viewModel is set up as follows:
var viewModel = {
a: ko.observable(0),
b: ko.observable(1),
c: ko.observable(2),
// commented this out, since
// the dependent observable will handle this
// d: ko.observable(0)
};
In my dependent observable:
viewModel.d = ko.dependentObservable(function () {
var theResult = 0;
$('.theLabel').css("visibility", "visible");
theResult=viewModel.a() * viewModel.b() * viewModel.c();
// if we return here we get a valid result
return (theResult);
// prefer to call ajax method
// first check to ensure one variable is set
if (viewModel.a() > 0) {
$.ajax("/myCalculation/getResult", {
data: ko.toJSON(viewModel),
type: "post",
context: viewModel,
contentType: "application/json",
success: function (result) {
// can't set visibility here
$('.theLabel').css("visibility", "visible");
// the POST does not pick up some observables, or
// does not the set dependent observable at all
return result;
}
});
}
});
There is quite a bit wrong with the function you have setup.
1.) You are returning from your function before making your AJAX call. The code after your return statement will never execute.
2.) Even if you omit the first return statement, your AJAX call is Asynchronous... which means it will execute in the background and return control to the outer scope immediately. Since you don't have a return statement, then you are going to return undefined.
3.) The comment in your success callback suggests you are expecting the return statement to propagate all the way up to your computed observable. THAT return statement is only scoped to the callback, and not the outer observable. The return value will be used by jQuery, and your observable will long since have returned.
If you want an observable to call an AJAX function you need a seperate value to store the results of the asynchronous call.
Here is a simplified example:
var viewModel = function(){
var $this = this;
$this.a = ko.observable();
$this.b = ko.observable();
$this.results = ko.observable();
//No need to assign this computed observable to a variable
// because the results will be stored in '$this.results'
// we just need this to handle the automatic updates
ko.computed(function(){
var data = {
a: $this.a(),
b: $this.b()
};
$.post("/do/some/stuff", data, function(results){
$this.results(results);
});
});
};