I'm looking to use sinon stub to test the method chain below:
driver.manage().window().setSize()
I found a related question that explains how to access one method down the chain, however this does not seem to give me access to any additional methods.
t.context.webdriver = sinon.stub(new WebDriver)
sinon.stub(t.context.webdriver, "manage", () => {
return {
window: sinon.stub().returns();
};
})
which returns the error
Error: this._driver.manage(...).window(...).setSize is not a function
How to I stub multi-level method chains?
I'm not sure what your trying to to test, but the error is coming from the fact that the object your stub is returning doesn't have a window() function or a setSize(). Chains work because each part of the chain returns something with a method that matches the next call. So if you stuff something early in the chain, you need to makes sure what you return has those methods. Maybe that involves passing back the original return, or maybe you fake the whole chain.
Here's an example that at least won't throw:
const sinon = require('sinon')
// some fake object that maybe looks like what you have
let driver = {
manage(){ return this},
window() { return this},
setSize() {console.log("size set")}
}
// stubb manage and now you're resposible for the whole chain
sinon.stub(driver, "manage").callsFake(() => {
console.log("called")
return {
window(){
return { setSize: sinon.stub().returns() }
}
};
})
Of course, there are a lot of variations possible depending on what you're trying to test.
Related
I have a one method that looks like this:
doSomething(){
return somethingPromisy().then((blerp) => {
// do something with blerp
return blerp; // Modified, of course
});
};
Then I have another method that looks like this:
doSomethingElse(){
stepOne();
var x = stepTwo();
var y = stepThree(x);
doSomething.then((data) => {
stepFour(data + y);
});
};
I'm using mocha+chai+sinon to test this code, in particular, doSomethingElse, and then I want to make some assertions - but how can I guarantee that the promise will be resolved by the time I make assertions? I know one option would be to change it to:
doSomethingElse(){
/* ... */
return doSomething.then(...);
};
If I do this, then it's pretty easy to write my test because then it's:
return doSomethingElse().then(() => {
someFake.lastCall.args.should.deep.equal(expectedData);
});
In my test and everything is fine. But should I be returning the promise simply for the sake of returning the promise? I don't actually care about any kind of return value from doSomethingElse - I only care that when I'm testing that the function in doSomething.then was called, i.e. the doSomething promise has been resolved by the time I do my assertions.
So what's the "best" way to go about this?
You should be testing doSomething and doSomethingElse separately with doSomethingElse using a mock of doSomething, with a mocked then methd that can return your value for testing. You basically want to test them separately not at the same time. You're basically coupling doSomething and doSomethingElse in this case which makes testing harder. Testing should be easy, you aren't testing to see if promises work, so use a fake one to move on to testing logic!
I do not find a lack of return value to be an issue myself, there is a return value, you're just chaining it, not storing it.
i write this code but i don`t now why my console.log give me undefined and how can i get value of prototype properties or functions
(function() {
function Users() {}
Users.prototype.getUser = function() {
$.getJSON('/usersList/users.json', function(request) {
Users.prototype.allUsers = request.users;
});
}
var users = new Users();
users.getUsers();
console.log(users.allUsers);
}
())
What i wont to achieve is to have this user list as my object property like User.allUsers in some array.
Thanks
This is because $.getJSON is asynchronous.
When you create an object from User prototype, $.getJSON has to call its callback yet.
In the other hand, you're trying to simulate global variables adding data properties to a given prototype. This is a bad practice and, at the end of the day, you've already realized that you got stuck on this approach.
What you need an initialization code executed whenever the site or app is started so those allUsers are called once and injected everywhere:
const initApp = () => Promise.all([
$.getJSON("/someData/users.json")
// other comma-separated async functions which return promises
])
initApp().then(([allUsers, otherData, yetAnotherData]) => {
callSomeFunctionWhichRequiresAllUsers(allUsers)
})
I'm trying to create a constructor for a blogging platform and it has many async operations going on inside. These range from grabbing the posts from directories, parsing them, sending them through template engines, etc.
So my question is, would it be unwise to have my constructor function return a promise instead of an object of the function they called new against.
For instance:
var engine = new Engine({path: '/path/to/posts'}).then(function (eng) {
// allow user to interact with the newly created engine object inside 'then'
engine.showPostsOnOnePage();
});
Now, the user may also not supply a supplement Promise chain link:
var engine = new Engine({path: '/path/to/posts'});
// ERROR
// engine will not be available as an Engine object here
This could pose a problem as the user may be confused why engine is not available after construction.
The reason to use a Promise in the constructor makes sense. I want the entire blog to be functioning after the construction phase. However, it seems like a smell almost to not have access to the object immediately after calling new.
I have debated using something along the lines of engine.start().then() or engine.init() which would return the Promise instead. But those also seem smelly.
Edit: This is in a Node.js project.
Yes, it is a bad practise. A constructor should return an instance of its class, nothing else. It would mess up the new operator and inheritance otherwise.
Moreover, a constructor should only create and initialize a new instance. It should set up data structures and all instance-specific properties, but not execute any tasks. It should be a pure function without side effects if possible, with all the benefits that has.
What if I want to execute things from my constructor?
That should go in a method of your class. You want to mutate global state? Then call that procedure explicitly, not as a side effect of generating an object. This call can go right after the instantiation:
var engine = new Engine()
engine.displayPosts();
If that task is asynchronous, you can now easily return a promise for its results from the method, to easily wait until it is finished.
I would however not recommend this pattern when the method (asynchronously) mutates the instance and other methods depend on that, as that would lead to them being required to wait (become async even if they're actually synchronous) and you'd quickly have some internal queue management going on. Do not code instances to exist but be actually unusable.
What if I want to load data into my instance asynchronously?
Ask yourself: Do you actually need the instance without the data? Could you use it somehow?
If the answer to that is No, then you should not create it before you have the data. Make the data ifself a parameter to your constructor, instead of telling the constructor how to fetch the data (or passing a promise for the data).
Then, use a static method to load the data, from which you return a promise. Then chain a call that wraps the data in a new instance on that:
Engine.load({path: '/path/to/posts'}).then(function(posts) {
new Engine(posts).displayPosts();
});
This allows much greater flexibility in the ways to acquire the data, and simplifies the constructor a lot. Similarly, you might write static factory functions that return promises for Engine instances:
Engine.fromPosts = function(options) {
return ajax(options.path).then(Engine.parsePosts).then(function(posts) {
return new Engine(posts, options);
});
};
…
Engine.fromPosts({path: '/path/to/posts'}).then(function(engine) {
engine.registerWith(framework).then(function(framePage) {
engine.showPostsOn(framePage);
});
});
I encountered the same problem and came up with this simple solution.
Instead of returning a Promise from the constructor, put it in this._initialized property, like this:
function Engine(path) {
this._initialized = Promise.resolve()
.then(() => {
return doSomethingAsync(path)
})
.then((result) => {
this.resultOfAsyncOp = result
})
}
Then, wrap every method in a callback that runs after the initialization, like that:
Engine.prototype.showPostsOnPage = function () {
return this._initialized.then(() => {
// actual body of the method
})
}
How it looks from the API consumer perspective:
engine = new Engine({path: '/path/to/posts'})
engine.showPostsOnPage()
This works because you can register multiple callbacks to a promise and they run either after it resolves or, if it's already resolved, at the time of attaching the callback.
This is how mongoskin works, except it doesn't actually use promises.
Edit: Since I wrote that reply I've fallen in love with ES6/7 syntax so there's another example using that.
class Engine {
constructor(path) {
this._initialized = this._initialize(path)
}
async _initialize() {
// actual async constructor logic
this.resultOfAsyncOp = await doSomethingAsync(path)
}
async showPostsOnPage() {
await this._initialized
// actual body of the method
}
}
To avoid the separation of concerns, use a factory to create the object.
class Engine {
constructor(data) {
this.data = data;
}
static makeEngine(pathToData) {
return new Promise((resolve, reject) => {
getData(pathToData).then(data => {
resolve(new Engine(data))
}).catch(reject);
});
}
}
The return value from the constructor replaces the object that the new operator just produced, so returning a promise is not a good idea. Previously, an explicit return value from the constructor was used for the singleton pattern.
The better way in ECMAScript 2017 is to use a static methods: you have one process, which is the numerality of static.
Which method to be run on the new object after the constructor may be known only to the class itself. To encapsulate this inside the class, you can use process.nextTick or Promise.resolve, postponing further execution allowing for listeners to be added and other things in Process.launch, the invoker of the constructor.
Since almost all code executes inside of a Promise, errors will end up in Process.fatal
This basic idea can be modified to fit specific encapsulation needs.
class MyClass {
constructor(o) {
if (o == null) o = false
if (o.run) Promise.resolve()
.then(() => this.method())
.then(o.exit).catch(o.reject)
}
async method() {}
}
class Process {
static launch(construct) {
return new Promise(r => r(
new construct({run: true, exit: Process.exit, reject: Process.fatal})
)).catch(Process.fatal)
}
static exit() {
process.exit()
}
static fatal(e) {
console.error(e.message)
process.exit(1)
}
}
Process.launch(MyClass)
This is in typescript, but should be easily converted to ECMAscript
export class Cache {
private aPromise: Promise<X>;
private bPromise: Promise<Y>;
constructor() {
this.aPromise = new Promise(...);
this.bPromise = new Promise(...);
}
public async saveFile: Promise<DirectoryEntry> {
const aObject = await this.aPromise;
// ...
}
}
The general pattern is to store the promises as internal variables using the constructor and await for the promises in the methods and make the methods all return promises. This allows you to use async/await to avoid long promise chains.
The example I gave is good enough for short promises, but putting in something that requires a long promise chain will make this messy, so to avoid that create a private async method that will be called by the constructor.
export class Cache {
private aPromise: Promise<X>;
private bPromise: Promise<Y>;
constructor() {
this.aPromise = initAsync();
this.bPromise = new Promise(...);
}
public async saveFile: Promise<DirectoryEntry> {
const aObject = await this.aPromise;
// ...
}
private async initAsync() : Promise<X> {
// ...
}
}
Here is a more fleshed out example for Ionic/Angular
import { Injectable } from "#angular/core";
import { DirectoryEntry, File } from "#ionic-native/file/ngx";
#Injectable({
providedIn: "root"
})
export class Cache {
private imageCacheDirectoryPromise: Promise<DirectoryEntry>;
private pdfCacheDirectoryPromise: Promise<DirectoryEntry>;
constructor(
private file: File
) {
this.imageCacheDirectoryPromise = this.initDirectoryEntry("image-cache");
this.pdfCacheDirectoryPromise = this.initDirectoryEntry("pdf-cache");
}
private async initDirectoryEntry(cacheDirectoryName: string): Promise<DirectoryEntry> {
const cacheDirectoryEntry = await this.resolveLocalFileSystemDirectory(this.file.cacheDirectory);
return this.file.getDirectory(cacheDirectoryEntry as DirectoryEntry, cacheDirectoryName, { create: true })
}
private async resolveLocalFileSystemDirectory(path: string): Promise<DirectoryEntry> {
const entry = await this.file.resolveLocalFilesystemUrl(path);
if (!entry.isDirectory) {
throw new Error(`${path} is not a directory`)
} else {
return entry as DirectoryEntry;
}
}
public async imageCacheDirectory() {
return this.imageCacheDirectoryPromise;
}
public async pdfCacheDirectory() {
return this.pdfCacheDirectoryPromise;
}
}
I run into this every now and then:
return somethingThatReturnsAPromise()
.then((response) => {
soSomethingg(); // Eg; update the UI
return response;
});
Now I'm looking for something that is not expected to return anything and won't change the promise chain if I forget that:
return somethingThatReturnsAPromise()
.whatImLookingFor((response) => {
doSomething(); // Eg; update the UI
})
.then((response) => {
// and this one should still be able to access response
});
Maybe this goes against the idea of promises, but for me, it's a bit inconvenient since I can't pass arbitrary functions.
One idea is to compose a function:
const sideEffect = (callback) => {
return (response) => {
callback(response);
return response;
};
};
And I could use it as
return somethingThatReturnsAPromise()
.then(sideEffect(doSomething));
But I'd prefer something instead of then is there something like that?
Note: I'm working with Angular 1.x so I need something like for that.
I would assume that you're not really writing .then().then(), because you could collapse that into a single .then, but that your concern is really about returning the promise and having some external code add another then to the chain. In that case do this:
let p = somethingThatReturnsAPromise();
p.then(() => doSomething());
return p;
This allows the caller to attach additional thens to the original promise instead of chaining off of your .then, thereby receiving the original promise's value. This is called branching the promise chain.
Maybe this goes against the idea of promises
Slightly, promise chains are pipelines where then handlers transform things at each stage. But it's perfectly valid to want to pass through the value unchanged.
One idea is to compose a function:
Indeed the first thing that came to mind, and how I'd do it.
But I'd prefer something instead of then is there something like that?
There isn't. You could add it for your own projects (I wouldn't in a library) by adding it to Promise.prototype. Or you could give yourselve a Promise subclass and add it there.
With a Promise sublass you'd do something like:
return MyPromise.resolve(somethingThatReturnsAPromise())
.thenSide(soSomethingg); // Eg; update the UI
...where thenSide is your method that's then but passing the original value back unchanged, e.g.:
class MyPromise extends Promise {
thenSide(callback) {
this.then(callback);
return this;
}
}
or
class MyPromise extends Promise {
thenSide(callback) {
this.then(callback);
return MyPromise.resolve(this);
}
}
...depending on whether you're bothered about thenSide returning the same promise (since then always returns a new one).
As far as I know (I could well be wrong) the wrapper method for "pass-through" side-effects is an idiomatic way to do what you want.
Alternatively (if you need the same response in multiple places) you can break up the promise chain when you encounter a situation like this.
There are three seperate questions that are similar to this one but none of them resembles the case I have.
So I basically have a function which takes a function as a parameter
var myfunc ( func_outer ) {
return func_outer().func_inner();
}
In my unit tests I want to be able to make a stub of a myfunc2. Basically I need to be able to stub a stub which is a nested stub. I currently use this kind of a manual stub but I would rather do it using sinon stubs if there is a way.
const func_outer = () => {
return {
func_inner: () => {return mockResponse;}
}
};
Has anyone ever faced this situation. Is there an easy way to solve this issue?
From sinon documentation you can check the returns section
stub.returns(obj);
Makes the stub return the provided value.
You can try the following:
First you should make sure that you stub your inner function, and then make it return the value you want.
func_innerStub = sinon.stub().returns('mockResponse')
Then stub your outer function and make it return the object with your stubbed inner function.
func_outerStub = sinon.stub().returns({func_inner: func_innerStub})
You can follow this pattern with the myfunc function, as well and pass as a param the func_outerStub.