How to inject conditional checks before execution of any class methods - javascript

I'm looking for some opinions / solutions on how to handle async data retrieval gracefully.
In initializing any class with some data asynchronously, I've been taking the approach like so:
class SomeClass {
// Turning off ts compiler's strictPropertyInitialization
private someProperty: SomeType
public async init(): Promise<this> {
this.someProperty = await goAndGetDataFromWhoKnowsWhere();
return this;
}
public async aMethod(): Promise<AType> {
// do its thing
}
public async anotherMethod(): Promise<AnotherType> {
// do its thing
}
}
And expect the user (myself / another co-worker) to use this class like so:
const someResult = new SomeClass()
.init()
.then( thatClass => thatClass.aMethod() )
This approach sure serves the purpose okay, but there're no hard restrictions to make sure the init() is getting called. Sometimes when someone forgets about it things break.
We could probably turn on the strictPropertyInitialization and inject checking in every single class method. That'd work for sure but the identical lines in the methods are yelling that there's probably a better way.
class SomeClass {
private someProperty: SomeType | undefined // To enforce null-checking
public async init(): Promise<this> {
this.someProperty = await goAndGetDataFromWhoKnowsWhere();
return this;
}
public async aMethod(): Promise<AType> {
if (!this.someProperty) await this.init();
// do its thing
}
public async anotherMethod(): Promise<AnotherType> {
if (!this.someProperty) await this.init();
// do its thing
}
}
Are there any solutions to this problem? Any design patterns that work around this problem? Helps appreciated! :)

Have you thought about not exposing the new() constructor call at all? What if you make the constructor private, and expose a static init() method that asynchronously constructs an instance and fills it with the data:
class SomeClass {
static async init(): Promise<SomeClass> {
return new SomeClass(await goAndGetDataFromWhoKnowsWhere());
}
private constructor(private someProperty: SomeType) { }
// your other methods
}
new SomeClass("oops"); // can't do this
SomeClass.init().then(thatClass => thatClass.aMethod());
Now it's essentially impossible for anyone to use this the wrong way. Hope that gives you some ideas. Good luck!

Another option is you could wrap creation of the class in a function. Assuming init must be called on every instance, you could just handle that on creation:
(Sorry it's not in TypeScript; I'm just not as familiar with it.)
const goAndGetDataFromWhoKnowsWhere = async () => 123;
const SomeClass = () => {
class SomeClass {
async init() {
this.someProperty = await goAndGetDataFromWhoKnowsWhere();
return this;
}
}
return new SomeClass().init();
};
SomeClass().then(inst => {
console.log('someProperty:', inst.someProperty);
});
Similar to jcalz's answer this would not allow use with the new keyword:
new SomeClass(); // TypeError: SomeClass is not a constructor

How about just using a function instead?
function SomeClass(){
var newObj = Object.create(/* your prototype */)
return goAndGetDataFromWhoKnowsWhere()
.then((data) => {
newObj.someProperty = data;
return newObj;
})
}
SomeClass().then((newObj) => {})

Related

Async function call inside a constructor in javascript class [duplicate]

At the moment, I'm attempting to use async/await within a class constructor function. This is so that I can get a custom e-mail tag for an Electron project I'm working on.
customElements.define('e-mail', class extends HTMLElement {
async constructor() {
super()
let uid = this.getAttribute('data-uid')
let message = await grabUID(uid)
const shadowRoot = this.attachShadow({mode: 'open'})
shadowRoot.innerHTML = `
<div id="email">A random email message has appeared. ${message}</div>
`
}
})
At the moment however, the project does not work, with the following error:
Class constructor may not be an async method
Is there a way to circumvent this so that I can use async/await within this? Instead of requiring callbacks or .then()?
This can never work.
The async keyword allows await to be used in a function marked as async but it also converts that function into a promise generator. So a function marked with async will return a promise. A constructor on the other hand returns the object it is constructing. Thus we have a situation where you want to both return an object and a promise: an impossible situation.
You can only use async/await where you can use promises because they are essentially syntax sugar for promises. You can't use promises in a constructor because a constructor must return the object to be constructed, not a promise.
There are two design patterns to overcome this, both invented before promises were around.
Use of an init() function. This works a bit like jQuery's .ready(). The object you create can only be used inside its own init or ready function:
Usage:
var myObj = new myClass();
myObj.init(function() {
// inside here you can use myObj
});
Implementation:
class myClass {
constructor () {
}
init (callback) {
// do something async and call the callback:
callback.bind(this)();
}
}
Use a builder. I've not seen this used much in javascript but this is one of the more common work-arounds in Java when an object needs to be constructed asynchronously. Of course, the builder pattern is used when constructing an object that requires a lot of complicated parameters. Which is exactly the use-case for asynchronous builders. The difference is that an async builder does not return an object but a promise of that object:
Usage:
myClass.build().then(function(myObj) {
// myObj is returned by the promise,
// not by the constructor
// or builder
});
// with async/await:
async function foo () {
var myObj = await myClass.build();
}
Implementation:
class myClass {
constructor (async_param) {
if (typeof async_param === 'undefined') {
throw new Error('Cannot be called directly');
}
}
static build () {
return doSomeAsyncStuff()
.then(function(async_result){
return new myClass(async_result);
});
}
}
Implementation with async/await:
class myClass {
constructor (async_param) {
if (typeof async_param === 'undefined') {
throw new Error('Cannot be called directly');
}
}
static async build () {
var async_result = await doSomeAsyncStuff();
return new myClass(async_result);
}
}
Note: although in the examples above we use promises for the async builder they are not strictly speaking necessary. You can just as easily write a builder that accept a callback.
Note on calling functions inside static functions.
This has nothing whatsoever to do with async constructors but with what the keyword this actually mean (which may be a bit surprising to people coming from languages that do auto-resolution of method names, that is, languages that don't need the this keyword).
The this keyword refers to the instantiated object. Not the class. Therefore you cannot normally use this inside static functions since the static function is not bound to any object but is bound directly to the class.
That is to say, in the following code:
class A {
static foo () {}
}
You cannot do:
var a = new A();
a.foo() // NOPE!!
instead you need to call it as:
A.foo();
Therefore, the following code would result in an error:
class A {
static foo () {
this.bar(); // you are calling this as static
// so bar is undefinned
}
bar () {}
}
To fix it you can make bar either a regular function or a static method:
function bar1 () {}
class A {
static foo () {
bar1(); // this is OK
A.bar2(); // this is OK
}
static bar2 () {}
}
You can definitely do this, by returning an Immediately Invoked Async Function Expression from the constructor. IIAFE is the fancy name for a very common pattern that was required in order to use await outside of an async function, before top-level await became available:
(async () => {
await someFunction();
})();
We'll be using this pattern to immediately execute the async function in the constructor, and return its result as this:
// Sample async function to be used in the async constructor
async function sleep(ms) {
return new Promise(resolve => setTimeout(resolve, ms));
}
class AsyncConstructor {
constructor(value) {
return (async () => {
// Call async functions here
await sleep(500);
this.value = value;
// Constructors return `this` implicitly, but this is an IIFE, so
// return `this` explicitly (else we'd return an empty object).
return this;
})();
}
}
(async () => {
console.log('Constructing...');
const obj = await new AsyncConstructor(123);
console.log('Done:', obj);
})();
To instantiate the class, use:
const instance = await new AsyncConstructor(...);
For TypeScript, you need to assert that the type of the constructor is the class type, rather than a promise returning the class type:
class AsyncConstructor {
constructor(value) {
return (async (): Promise<AsyncConstructor> => {
// ...
return this;
})() as unknown as AsyncConstructor; // <-- type assertion
}
}
Downsides
Extending a class with an async constructor will have a limitation. If you need to call super in the constructor of the derived class, you'll have to call it without await. If you need to call the super constructor with await, you'll run into TypeScript error 2337: Super calls are not permitted outside constructors or in nested functions inside constructors.
It's been argued that it's a "bad practice" to have a constructor function return a Promise.
Before using this solution, determine whether you'll need to extend the class, and document that the constructor must be called with await.
Because async functions are promises, you can create a static function on your class which executes an async function which returns the instance of the class:
class Yql {
constructor () {
// Set up your class
}
static init () {
return (async function () {
let yql = new Yql()
// Do async stuff
await yql.build()
// Return instance
return yql
}())
}
async build () {
// Do stuff with await if needed
}
}
async function yql () {
// Do this instead of "new Yql()"
let yql = await Yql.init()
// Do stuff with yql instance
}
yql()
Call with let yql = await Yql.init() from an async function.
Unlike others have said, you can get it to work.
JavaScript classes can return literally anything from their constructor, even an instance of another class. So, you might return a Promise from the constructor of your class that resolves to its actual instance.
Below is an example:
export class Foo {
constructor() {
return (async () => {
// await anything you want
return this; // Return the newly-created instance
})();
}
}
Then, you'll create instances of Foo this way:
const foo = await new Foo();
The stopgap solution
You can create an async init() {... return this;} method, then instead do new MyClass().init() whenever you'd normally just say new MyClass().
This is not clean because it relies on everyone who uses your code, and yourself, to always instantiate the object like so. However if you're only using this object in a particular place or two in your code, it could maybe be fine.
A significant problem though occurs because ES has no type system, so if you forget to call it, you've just returned undefined because the constructor returns nothing. Oops. Much better would be to do something like:
The best thing to do would be:
class AsyncOnlyObject {
constructor() {
}
async init() {
this.someField = await this.calculateStuff();
}
async calculateStuff() {
return 5;
}
}
async function newAsync_AsyncOnlyObject() {
return await new AsyncOnlyObject().init();
}
newAsync_AsyncOnlyObject().then(console.log);
// output: AsyncOnlyObject {someField: 5}
The factory method solution (slightly better)
However then you might accidentally do new AsyncOnlyObject, you should probably just create factory function that uses Object.create(AsyncOnlyObject.prototype) directly:
async function newAsync_AsyncOnlyObject() {
return await Object.create(AsyncOnlyObject.prototype).init();
}
newAsync_AsyncOnlyObject().then(console.log);
// output: AsyncOnlyObject {someField: 5}
However say you want to use this pattern on many objects... you could abstract this as a decorator or something you (verbosely, ugh) call after defining like postProcess_makeAsyncInit(AsyncOnlyObject), but here I'm going to use extends because it sort of fits into subclass semantics (subclasses are parent class + extra, in that they should obey the design contract of the parent class, and may do additional things; an async subclass would be strange if the parent wasn't also async, because it could not be initialized the same way):
Abstracted solution (extends/subclass version)
class AsyncObject {
constructor() {
throw new Error('classes descended from AsyncObject must be initialized as (await) TheClassName.anew(), rather than new TheClassName()');
}
static async anew(...args) {
var R = Object.create(this.prototype);
R.init(...args);
return R;
}
}
class MyObject extends AsyncObject {
async init(x, y=5) {
this.x = x;
this.y = y;
// bonus: we need not return 'this'
}
}
MyObject.anew('x').then(console.log);
// output: MyObject {x: "x", y: 5}
(do not use in production: I have not thought through complicated scenarios such as whether this is the proper way to write a wrapper for keyword arguments.)
Based on your comments, you should probably do what every other HTMLElement with asset loading does: make the constructor start a sideloading action, generating a load or error event depending on the result.
Yes, that means using promises, but it also means "doing things the same way as every other HTML element", so you're in good company. For instance:
var img = new Image();
img.onload = function(evt) { ... }
img.addEventListener("load", evt => ... );
img.onerror = function(evt) { ... }
img.addEventListener("error", evt => ... );
img.src = "some url";
this kicks off an asynchronous load of the source asset that, when it succeeds, ends in onload and when it goes wrong, ends in onerror. So, make your own class do this too:
class EMailElement extends HTMLElement {
connectedCallback() {
this.uid = this.getAttribute('data-uid');
}
setAttribute(name, value) {
super.setAttribute(name, value);
if (name === 'data-uid') {
this.uid = value;
}
}
set uid(input) {
if (!input) return;
const uid = parseInt(input);
// don't fight the river, go with the flow, use a promise:
new Promise((resolve, reject) => {
yourDataBase.getByUID(uid, (err, result) => {
if (err) return reject(err);
resolve(result);
});
})
.then(result => {
this.renderLoaded(result.message);
})
.catch(error => {
this.renderError(error);
});
}
};
customElements.define('e-mail', EmailElement);
And then you make the renderLoaded/renderError functions deal with the event calls and shadow dom:
renderLoaded(message) {
const shadowRoot = this.attachShadow({mode: 'open'});
shadowRoot.innerHTML = `
<div class="email">A random email message has appeared. ${message}</div>
`;
// is there an ancient event listener?
if (this.onload) {
this.onload(...);
}
// there might be modern event listeners. dispatch an event.
this.dispatchEvent(new Event('load'));
}
renderFailed() {
const shadowRoot = this.attachShadow({mode: 'open'});
shadowRoot.innerHTML = `
<div class="email">No email messages.</div>
`;
// is there an ancient event listener?
if (this.onload) {
this.onerror(...);
}
// there might be modern event listeners. dispatch an event.
this.dispatchEvent(new Event('error'));
}
Also note I changed your id to a class, because unless you write some weird code to only ever allow a single instance of your <e-mail> element on a page, you can't use a unique identifier and then assign it to a bunch of elements.
I usually prefer a static async method that returns a new instance, but here's another way to do it. It's closer to literally awaiting a constructor. It works with TypeScript.
class Foo {
#promiseReady;
constructor() {
this.#promiseReady = this.#init();
}
async #init() {
await someAsyncStuff();
return this;
}
ready() {
return this.promiseReady;
}
}
let foo = await new Foo().ready();
I made this test-case based on #Downgoat's answer.
It runs on NodeJS.
This is Downgoat's code where the async part is provided by a setTimeout() call.
'use strict';
const util = require( 'util' );
class AsyncConstructor{
constructor( lapse ){
this.qqq = 'QQQ';
this.lapse = lapse;
return ( async ( lapse ) => {
await this.delay( lapse );
return this;
})( lapse );
}
async delay(ms) {
return await new Promise(resolve => setTimeout(resolve, ms));
}
}
let run = async ( millis ) => {
// Instatiate with await, inside an async function
let asyncConstructed = await new AsyncConstructor( millis );
console.log( 'AsyncConstructor: ' + util.inspect( asyncConstructed ));
};
run( 777 );
My use case is DAOs for the server-side of a web application.
As I see DAOs, they are each one associated to a record format, in my case a MongoDB collection like for instance a cook.
A cooksDAO instance holds a cook's data.
In my restless mind I would be able to instantiate a cook's DAO providing the cookId as an argument, and the instantiation would create the object and populate it with the cook's data.
Thus the need to run async stuff into the constructor.
I wanted to write:
let cook = new cooksDAO( '12345' );
to have available properties like cook.getDisplayName().
With this solution I have to do:
let cook = await new cooksDAO( '12345' );
which is very similar to the ideal.
Also, I need to do this inside an async function.
My B-plan was to leave the data loading out of the constructor, based on #slebetman suggestion to use an init function, and do something like this:
let cook = new cooksDAO( '12345' );
async cook.getData();
which doesn't break the rules.
Use the async method in constructor???
constructor(props) {
super(props);
(async () => await this.qwe(() => console.log(props), () => console.log(props)))();
}
async qwe(q, w) {
return new Promise((rs, rj) => {
rs(q());
rj(w());
});
}
If you can avoid extend, you can avoid classes all together and use function composition as constructors. You can use the variables in the scope instead of class members:
async function buildA(...) {
const data = await fetch(...);
return {
getData: function() {
return data;
}
}
}
and simple use it as
const a = await buildA(...);
If you're using typescript or flow, you can even enforce the interface of the constructors
Interface A {
getData: object;
}
async function buildA0(...): Promise<A> { ... }
async function buildA1(...): Promise<A> { ... }
...
I found myself in a situation like this and ended up using an IIFE
// using TypeScript
class SomeClass {
constructor() {
// do something here
}
doSomethingAsync(): SomeClass {
(async () => await asyncTask())();
return this;
}
}
const someClass = new SomeClass().doSomethingAsync();
If you have other tasks that are dependant on the async task you can run them after the IIFE completes its execution.
A lot of great knowledge here and some super() thoughtful responses. In short the technique outlined below is fairly straightforward, non-recursive, async-compatible and plays by the rules. More importantly I don't believe it has been properly covered here yet - though please correct me if wrong!
Instead of method calls we simply assign an II(A)FE to an instance prop:
// it's async-lite!
class AsyncLiteComponent {
constructor() {
// our instance includes a 'ready' property: an IIAFE promise
// that auto-runs our async needs and then resolves to the instance
// ...
// this is the primary difference to other answers, in that we defer
// from a property, not a method, and the async functionality both
// auto-runs and the promise/prop resolves to the instance
this.ready = (async () => {
// in this example we're auto-fetching something
this.msg = await AsyncLiteComponent.msg;
// we return our instance to allow nifty one-liners (see below)
return this;
})();
}
// we keep our async functionality in a static async getter
// ... technically (with some minor tweaks), we could prefetch
// or cache this response (but that isn't really our goal here)
static get msg() {
// yes I know - this example returns almost immediately (imagination people!)
return fetch('data:,Hello%20World%21').then((e) => e.text());
}
}
Seems simple enough, how is it used?
// Ok, so you *could* instantiate it the normal, excessively boring way
const iwillnotwait = new AsyncLiteComponent();
// and defer your waiting for later
await iwillnotwait.ready
console.log(iwillnotwait.msg)
// OR OR OR you can get all async/awaity about it!
const onlywhenimready = await new AsyncLiteComponent().ready;
console.log(onlywhenimready.msg)
// ... if you're really antsy you could even "pre-wait" using the static method,
// but you'd probably want some caching / update logic in the class first
const prefetched = await AsyncLiteComponent.msg;
// ... and I haven't fully tested this but it should also be open for extension
class Extensior extends AsyncLiteComponent {
constructor() {
super();
this.ready.then(() => console.log(this.msg))
}
}
const extendedwaittime = await new Extensior().ready;
Before posting I had a brief discussion on the viability of this technique in the comments of #slebetman's comprehensive answer. I wasn't entirely convinced by the outright dismissal, so thought I would open it up to further debate / tear down. Please do your worst :)
You can use Proxy's construct handle to do this, the code like this:
const SomeClass = new Proxy(class A {
constructor(user) {
this.user = user;
}
}, {
async construct(target, args, newTarget) {
const [name] = args;
// you can use await in here
const user = await fetch(name);
// invoke new A here
return new target(user);
}
});
const a = await new SomeClass('cage');
console.log(a.user); // user info
Variation on the builder pattern, using call():
function asyncMethod(arg) {
function innerPromise() { return new Promise((...)=> {...}) }
innerPromise().then(result => {
this.setStuff(result);
}
}
const getInstance = async (arg) => {
let instance = new Instance();
await asyncMethod.call(instance, arg);
return instance;
}
That can be done, like this:
class test
{
constructor () { return Promise.resolve (this); }
}
Or with real delays added, any asynchronous event, or setTimeout for instance:
class test
{
constructor ()
{
return new Promise ( (resolve, reject) =>
{ //doing something really delayed
setTimeout (resolve, 5, this);
});
}
doHello(a) {console.log("hello: " + a);}
}
async function main()
{
new test().then(a=> a.doHello("then")); //invoking asynchronously
console.log("testing"); //"testing" will be printed 5 seconds before "hello"
(await new test()).doHello("await"); //invoking synchronously
}
main();
In some cases, when involving inheritance, can't be returned Promise from base class constructor, it will mess with this pointer. First idea is adding function then to make the class promise alike, but it is problematic. My workaround is using a private variable #p retaining the promise, and function _then (instead of then) which should act on completion of #p
class basetest
{
#p = null;
constructor (){this.#p = Promise.resolve(this);}
async _then (func)
{
return this.#p.then ( (ths) =>
{
this.#p = null;
if (func) func (ths);
return this;
});
}
doHello(a) {console.log("hello: " + a);}
}
class test extends basetest
{
constructor(context)
{
super(context);
this.msg = "test: ";
}
doTest(a) {console.log(this.msg + a);}
}
async function main()
{
(await new test()._then()).doHello("await");//synchronous call
//note, now we use _then instead of then
(new test())._then(a=>{a.doHello("then");a.doTest("then");} );//asynchronous call
}
main();
And suppose a some webgl, with async image loading, generating a mesh and drawing a vertex array object:
class HeightMap extends GlVAObject
{
#vertices = [];
constructor (src, crossOrigin = "")
{
//super(theContextSetup);
let image = new Image();
image.src = src;
image.crossOrigin = crossOrigin;
return new Promise ( (resolve, reject) =>
{
image.addEventListener('load', () =>
{
//reading pixel values from image into this.#vertices
//and generate a heights map
//...
resolve(this);
} );
});
}
///...
}
function drawVao(vao) {/*do something*/}
async function main()
{
let vao = await new HeightMap ("./heightmaps/ArisonaCraterHeightMap.png");
drawVao(vao);
///...
}
/*
//version of main with asynchronous call
async function main()
{
new HeightMap ("./heightmaps/ArisonaCraterHeightMap.png").then (vao => drawVao(vao));
///...
}
*/
main();
Second variant with _then, as it really works in real life:
class HeightMap extends GlVAObject
{
#vertices = [];
constructor (src, crossOrigin = "")
{
//super(theContextSetup);
let image = new Image();
image.src = src;
image.crossOrigin = crossOrigin;
this.#p = Promise ( (resolve, reject) =>
{
image.addEventListener('load', () =>
{
//...
resolve(this);
} );
});
}
async _then (func)
{
return this.#p.then ( (ths) =>
{
this.#p = null;
if (func) func (ths);
return this;
});
}
///...
}
function drawVao(vao) {/*do something*/}
async function main()
{
new HeightMap ("./heightmaps/ArisonaCraterHeightMap.png")._then(vao => drawVao(vao) );
///...
}
/*
//version of main with synchronous call
async function main()
{
vao = (await new HeightMap ("./heightmaps/ArisonaCraterHeightMap.png"))._then();
drawVao(vao);
///...
}*/
main();
You may immediately invoke an anonymous async function that returns message and set it to the message variable. You might want to take a look at immediately invoked function expressions (IEFES), in case you are unfamiliar with this pattern. This will work like a charm.
var message = (async function() { return await grabUID(uid) })()
You should add then function to instance. Promise will recognize it as a thenable object with Promise.resolve automatically
const asyncSymbol = Symbol();
class MyClass {
constructor() {
this.asyncData = null
}
then(resolve, reject) {
return (this[asyncSymbol] = this[asyncSymbol] || new Promise((innerResolve, innerReject) => {
this.asyncData = { a: 1 }
setTimeout(() => innerResolve(this.asyncData), 3000)
})).then(resolve, reject)
}
}
async function wait() {
const asyncData = await new MyClass();
alert('run 3s later')
alert(asyncData.a)
}
#slebetmen's accepted answer explains well why this doesn't work. In addition to the two patterns presented in that answer, another option is to only access your async properties through a custom async getter. The constructor() can then trigger the async creation of the properties, but the getter then checks to see if the property is available before it uses or returns it.
This approach is particularly useful when you want to initialize a global object once on startup, and you want to do it inside a module. Instead of initializing in your index.js and passing the instance in the places that need it, simply require your module wherever the global object is needed.
Usage
const instance = new MyClass();
const prop = await instance.getMyProperty();
Implementation
class MyClass {
constructor() {
this.myProperty = null;
this.myPropertyPromise = this.downloadAsyncStuff();
}
async downloadAsyncStuff() {
// await yourAsyncCall();
this.myProperty = 'async property'; // this would instead by your async call
return this.myProperty;
}
getMyProperty() {
if (this.myProperty) {
return this.myProperty;
} else {
return this.myPropertyPromise;
}
}
}
The closest you can get to an asynchronous constructor is by waiting for it to finish executing if it hasn't already in all of its methods:
class SomeClass {
constructor() {
this.asyncConstructor = (async () => {
// Perform asynchronous operations here
})()
}
async someMethod() {
await this.asyncConstructor
// Perform normal logic here
}
}
The other answers are missing the obvious. Simply call an async function from your constructor:
constructor() {
setContentAsync();
}
async setContentAsync() {
let uid = this.getAttribute('data-uid')
let message = await grabUID(uid)
const shadowRoot = this.attachShadow({mode: 'open'})
shadowRoot.innerHTML = `
<div id="email">A random email message has appeared. ${message}</div>
`
}

typescript class with constructor calling an internal async function that initiliaze a property [duplicate]

At the moment, I'm attempting to use async/await within a class constructor function. This is so that I can get a custom e-mail tag for an Electron project I'm working on.
customElements.define('e-mail', class extends HTMLElement {
async constructor() {
super()
let uid = this.getAttribute('data-uid')
let message = await grabUID(uid)
const shadowRoot = this.attachShadow({mode: 'open'})
shadowRoot.innerHTML = `
<div id="email">A random email message has appeared. ${message}</div>
`
}
})
At the moment however, the project does not work, with the following error:
Class constructor may not be an async method
Is there a way to circumvent this so that I can use async/await within this? Instead of requiring callbacks or .then()?
This can never work.
The async keyword allows await to be used in a function marked as async but it also converts that function into a promise generator. So a function marked with async will return a promise. A constructor on the other hand returns the object it is constructing. Thus we have a situation where you want to both return an object and a promise: an impossible situation.
You can only use async/await where you can use promises because they are essentially syntax sugar for promises. You can't use promises in a constructor because a constructor must return the object to be constructed, not a promise.
There are two design patterns to overcome this, both invented before promises were around.
Use of an init() function. This works a bit like jQuery's .ready(). The object you create can only be used inside its own init or ready function:
Usage:
var myObj = new myClass();
myObj.init(function() {
// inside here you can use myObj
});
Implementation:
class myClass {
constructor () {
}
init (callback) {
// do something async and call the callback:
callback.bind(this)();
}
}
Use a builder. I've not seen this used much in javascript but this is one of the more common work-arounds in Java when an object needs to be constructed asynchronously. Of course, the builder pattern is used when constructing an object that requires a lot of complicated parameters. Which is exactly the use-case for asynchronous builders. The difference is that an async builder does not return an object but a promise of that object:
Usage:
myClass.build().then(function(myObj) {
// myObj is returned by the promise,
// not by the constructor
// or builder
});
// with async/await:
async function foo () {
var myObj = await myClass.build();
}
Implementation:
class myClass {
constructor (async_param) {
if (typeof async_param === 'undefined') {
throw new Error('Cannot be called directly');
}
}
static build () {
return doSomeAsyncStuff()
.then(function(async_result){
return new myClass(async_result);
});
}
}
Implementation with async/await:
class myClass {
constructor (async_param) {
if (typeof async_param === 'undefined') {
throw new Error('Cannot be called directly');
}
}
static async build () {
var async_result = await doSomeAsyncStuff();
return new myClass(async_result);
}
}
Note: although in the examples above we use promises for the async builder they are not strictly speaking necessary. You can just as easily write a builder that accept a callback.
Note on calling functions inside static functions.
This has nothing whatsoever to do with async constructors but with what the keyword this actually mean (which may be a bit surprising to people coming from languages that do auto-resolution of method names, that is, languages that don't need the this keyword).
The this keyword refers to the instantiated object. Not the class. Therefore you cannot normally use this inside static functions since the static function is not bound to any object but is bound directly to the class.
That is to say, in the following code:
class A {
static foo () {}
}
You cannot do:
var a = new A();
a.foo() // NOPE!!
instead you need to call it as:
A.foo();
Therefore, the following code would result in an error:
class A {
static foo () {
this.bar(); // you are calling this as static
// so bar is undefinned
}
bar () {}
}
To fix it you can make bar either a regular function or a static method:
function bar1 () {}
class A {
static foo () {
bar1(); // this is OK
A.bar2(); // this is OK
}
static bar2 () {}
}
You can definitely do this, by returning an Immediately Invoked Async Function Expression from the constructor. IIAFE is the fancy name for a very common pattern that was required in order to use await outside of an async function, before top-level await became available:
(async () => {
await someFunction();
})();
We'll be using this pattern to immediately execute the async function in the constructor, and return its result as this:
// Sample async function to be used in the async constructor
async function sleep(ms) {
return new Promise(resolve => setTimeout(resolve, ms));
}
class AsyncConstructor {
constructor(value) {
return (async () => {
// Call async functions here
await sleep(500);
this.value = value;
// Constructors return `this` implicitly, but this is an IIFE, so
// return `this` explicitly (else we'd return an empty object).
return this;
})();
}
}
(async () => {
console.log('Constructing...');
const obj = await new AsyncConstructor(123);
console.log('Done:', obj);
})();
To instantiate the class, use:
const instance = await new AsyncConstructor(...);
For TypeScript, you need to assert that the type of the constructor is the class type, rather than a promise returning the class type:
class AsyncConstructor {
constructor(value) {
return (async (): Promise<AsyncConstructor> => {
// ...
return this;
})() as unknown as AsyncConstructor; // <-- type assertion
}
}
Downsides
Extending a class with an async constructor will have a limitation. If you need to call super in the constructor of the derived class, you'll have to call it without await. If you need to call the super constructor with await, you'll run into TypeScript error 2337: Super calls are not permitted outside constructors or in nested functions inside constructors.
It's been argued that it's a "bad practice" to have a constructor function return a Promise.
Before using this solution, determine whether you'll need to extend the class, and document that the constructor must be called with await.
Because async functions are promises, you can create a static function on your class which executes an async function which returns the instance of the class:
class Yql {
constructor () {
// Set up your class
}
static init () {
return (async function () {
let yql = new Yql()
// Do async stuff
await yql.build()
// Return instance
return yql
}())
}
async build () {
// Do stuff with await if needed
}
}
async function yql () {
// Do this instead of "new Yql()"
let yql = await Yql.init()
// Do stuff with yql instance
}
yql()
Call with let yql = await Yql.init() from an async function.
Unlike others have said, you can get it to work.
JavaScript classes can return literally anything from their constructor, even an instance of another class. So, you might return a Promise from the constructor of your class that resolves to its actual instance.
Below is an example:
export class Foo {
constructor() {
return (async () => {
// await anything you want
return this; // Return the newly-created instance
})();
}
}
Then, you'll create instances of Foo this way:
const foo = await new Foo();
The stopgap solution
You can create an async init() {... return this;} method, then instead do new MyClass().init() whenever you'd normally just say new MyClass().
This is not clean because it relies on everyone who uses your code, and yourself, to always instantiate the object like so. However if you're only using this object in a particular place or two in your code, it could maybe be fine.
A significant problem though occurs because ES has no type system, so if you forget to call it, you've just returned undefined because the constructor returns nothing. Oops. Much better would be to do something like:
The best thing to do would be:
class AsyncOnlyObject {
constructor() {
}
async init() {
this.someField = await this.calculateStuff();
}
async calculateStuff() {
return 5;
}
}
async function newAsync_AsyncOnlyObject() {
return await new AsyncOnlyObject().init();
}
newAsync_AsyncOnlyObject().then(console.log);
// output: AsyncOnlyObject {someField: 5}
The factory method solution (slightly better)
However then you might accidentally do new AsyncOnlyObject, you should probably just create factory function that uses Object.create(AsyncOnlyObject.prototype) directly:
async function newAsync_AsyncOnlyObject() {
return await Object.create(AsyncOnlyObject.prototype).init();
}
newAsync_AsyncOnlyObject().then(console.log);
// output: AsyncOnlyObject {someField: 5}
However say you want to use this pattern on many objects... you could abstract this as a decorator or something you (verbosely, ugh) call after defining like postProcess_makeAsyncInit(AsyncOnlyObject), but here I'm going to use extends because it sort of fits into subclass semantics (subclasses are parent class + extra, in that they should obey the design contract of the parent class, and may do additional things; an async subclass would be strange if the parent wasn't also async, because it could not be initialized the same way):
Abstracted solution (extends/subclass version)
class AsyncObject {
constructor() {
throw new Error('classes descended from AsyncObject must be initialized as (await) TheClassName.anew(), rather than new TheClassName()');
}
static async anew(...args) {
var R = Object.create(this.prototype);
R.init(...args);
return R;
}
}
class MyObject extends AsyncObject {
async init(x, y=5) {
this.x = x;
this.y = y;
// bonus: we need not return 'this'
}
}
MyObject.anew('x').then(console.log);
// output: MyObject {x: "x", y: 5}
(do not use in production: I have not thought through complicated scenarios such as whether this is the proper way to write a wrapper for keyword arguments.)
Based on your comments, you should probably do what every other HTMLElement with asset loading does: make the constructor start a sideloading action, generating a load or error event depending on the result.
Yes, that means using promises, but it also means "doing things the same way as every other HTML element", so you're in good company. For instance:
var img = new Image();
img.onload = function(evt) { ... }
img.addEventListener("load", evt => ... );
img.onerror = function(evt) { ... }
img.addEventListener("error", evt => ... );
img.src = "some url";
this kicks off an asynchronous load of the source asset that, when it succeeds, ends in onload and when it goes wrong, ends in onerror. So, make your own class do this too:
class EMailElement extends HTMLElement {
connectedCallback() {
this.uid = this.getAttribute('data-uid');
}
setAttribute(name, value) {
super.setAttribute(name, value);
if (name === 'data-uid') {
this.uid = value;
}
}
set uid(input) {
if (!input) return;
const uid = parseInt(input);
// don't fight the river, go with the flow, use a promise:
new Promise((resolve, reject) => {
yourDataBase.getByUID(uid, (err, result) => {
if (err) return reject(err);
resolve(result);
});
})
.then(result => {
this.renderLoaded(result.message);
})
.catch(error => {
this.renderError(error);
});
}
};
customElements.define('e-mail', EmailElement);
And then you make the renderLoaded/renderError functions deal with the event calls and shadow dom:
renderLoaded(message) {
const shadowRoot = this.attachShadow({mode: 'open'});
shadowRoot.innerHTML = `
<div class="email">A random email message has appeared. ${message}</div>
`;
// is there an ancient event listener?
if (this.onload) {
this.onload(...);
}
// there might be modern event listeners. dispatch an event.
this.dispatchEvent(new Event('load'));
}
renderFailed() {
const shadowRoot = this.attachShadow({mode: 'open'});
shadowRoot.innerHTML = `
<div class="email">No email messages.</div>
`;
// is there an ancient event listener?
if (this.onload) {
this.onerror(...);
}
// there might be modern event listeners. dispatch an event.
this.dispatchEvent(new Event('error'));
}
Also note I changed your id to a class, because unless you write some weird code to only ever allow a single instance of your <e-mail> element on a page, you can't use a unique identifier and then assign it to a bunch of elements.
I usually prefer a static async method that returns a new instance, but here's another way to do it. It's closer to literally awaiting a constructor. It works with TypeScript.
class Foo {
#promiseReady;
constructor() {
this.#promiseReady = this.#init();
}
async #init() {
await someAsyncStuff();
return this;
}
ready() {
return this.promiseReady;
}
}
let foo = await new Foo().ready();
I made this test-case based on #Downgoat's answer.
It runs on NodeJS.
This is Downgoat's code where the async part is provided by a setTimeout() call.
'use strict';
const util = require( 'util' );
class AsyncConstructor{
constructor( lapse ){
this.qqq = 'QQQ';
this.lapse = lapse;
return ( async ( lapse ) => {
await this.delay( lapse );
return this;
})( lapse );
}
async delay(ms) {
return await new Promise(resolve => setTimeout(resolve, ms));
}
}
let run = async ( millis ) => {
// Instatiate with await, inside an async function
let asyncConstructed = await new AsyncConstructor( millis );
console.log( 'AsyncConstructor: ' + util.inspect( asyncConstructed ));
};
run( 777 );
My use case is DAOs for the server-side of a web application.
As I see DAOs, they are each one associated to a record format, in my case a MongoDB collection like for instance a cook.
A cooksDAO instance holds a cook's data.
In my restless mind I would be able to instantiate a cook's DAO providing the cookId as an argument, and the instantiation would create the object and populate it with the cook's data.
Thus the need to run async stuff into the constructor.
I wanted to write:
let cook = new cooksDAO( '12345' );
to have available properties like cook.getDisplayName().
With this solution I have to do:
let cook = await new cooksDAO( '12345' );
which is very similar to the ideal.
Also, I need to do this inside an async function.
My B-plan was to leave the data loading out of the constructor, based on #slebetman suggestion to use an init function, and do something like this:
let cook = new cooksDAO( '12345' );
async cook.getData();
which doesn't break the rules.
Use the async method in constructor???
constructor(props) {
super(props);
(async () => await this.qwe(() => console.log(props), () => console.log(props)))();
}
async qwe(q, w) {
return new Promise((rs, rj) => {
rs(q());
rj(w());
});
}
If you can avoid extend, you can avoid classes all together and use function composition as constructors. You can use the variables in the scope instead of class members:
async function buildA(...) {
const data = await fetch(...);
return {
getData: function() {
return data;
}
}
}
and simple use it as
const a = await buildA(...);
If you're using typescript or flow, you can even enforce the interface of the constructors
Interface A {
getData: object;
}
async function buildA0(...): Promise<A> { ... }
async function buildA1(...): Promise<A> { ... }
...
I found myself in a situation like this and ended up using an IIFE
// using TypeScript
class SomeClass {
constructor() {
// do something here
}
doSomethingAsync(): SomeClass {
(async () => await asyncTask())();
return this;
}
}
const someClass = new SomeClass().doSomethingAsync();
If you have other tasks that are dependant on the async task you can run them after the IIFE completes its execution.
A lot of great knowledge here and some super() thoughtful responses. In short the technique outlined below is fairly straightforward, non-recursive, async-compatible and plays by the rules. More importantly I don't believe it has been properly covered here yet - though please correct me if wrong!
Instead of method calls we simply assign an II(A)FE to an instance prop:
// it's async-lite!
class AsyncLiteComponent {
constructor() {
// our instance includes a 'ready' property: an IIAFE promise
// that auto-runs our async needs and then resolves to the instance
// ...
// this is the primary difference to other answers, in that we defer
// from a property, not a method, and the async functionality both
// auto-runs and the promise/prop resolves to the instance
this.ready = (async () => {
// in this example we're auto-fetching something
this.msg = await AsyncLiteComponent.msg;
// we return our instance to allow nifty one-liners (see below)
return this;
})();
}
// we keep our async functionality in a static async getter
// ... technically (with some minor tweaks), we could prefetch
// or cache this response (but that isn't really our goal here)
static get msg() {
// yes I know - this example returns almost immediately (imagination people!)
return fetch('data:,Hello%20World%21').then((e) => e.text());
}
}
Seems simple enough, how is it used?
// Ok, so you *could* instantiate it the normal, excessively boring way
const iwillnotwait = new AsyncLiteComponent();
// and defer your waiting for later
await iwillnotwait.ready
console.log(iwillnotwait.msg)
// OR OR OR you can get all async/awaity about it!
const onlywhenimready = await new AsyncLiteComponent().ready;
console.log(onlywhenimready.msg)
// ... if you're really antsy you could even "pre-wait" using the static method,
// but you'd probably want some caching / update logic in the class first
const prefetched = await AsyncLiteComponent.msg;
// ... and I haven't fully tested this but it should also be open for extension
class Extensior extends AsyncLiteComponent {
constructor() {
super();
this.ready.then(() => console.log(this.msg))
}
}
const extendedwaittime = await new Extensior().ready;
Before posting I had a brief discussion on the viability of this technique in the comments of #slebetman's comprehensive answer. I wasn't entirely convinced by the outright dismissal, so thought I would open it up to further debate / tear down. Please do your worst :)
You can use Proxy's construct handle to do this, the code like this:
const SomeClass = new Proxy(class A {
constructor(user) {
this.user = user;
}
}, {
async construct(target, args, newTarget) {
const [name] = args;
// you can use await in here
const user = await fetch(name);
// invoke new A here
return new target(user);
}
});
const a = await new SomeClass('cage');
console.log(a.user); // user info
Variation on the builder pattern, using call():
function asyncMethod(arg) {
function innerPromise() { return new Promise((...)=> {...}) }
innerPromise().then(result => {
this.setStuff(result);
}
}
const getInstance = async (arg) => {
let instance = new Instance();
await asyncMethod.call(instance, arg);
return instance;
}
That can be done, like this:
class test
{
constructor () { return Promise.resolve (this); }
}
Or with real delays added, any asynchronous event, or setTimeout for instance:
class test
{
constructor ()
{
return new Promise ( (resolve, reject) =>
{ //doing something really delayed
setTimeout (resolve, 5, this);
});
}
doHello(a) {console.log("hello: " + a);}
}
async function main()
{
new test().then(a=> a.doHello("then")); //invoking asynchronously
console.log("testing"); //"testing" will be printed 5 seconds before "hello"
(await new test()).doHello("await"); //invoking synchronously
}
main();
In some cases, when involving inheritance, can't be returned Promise from base class constructor, it will mess with this pointer. First idea is adding function then to make the class promise alike, but it is problematic. My workaround is using a private variable #p retaining the promise, and function _then (instead of then) which should act on completion of #p
class basetest
{
#p = null;
constructor (){this.#p = Promise.resolve(this);}
async _then (func)
{
return this.#p.then ( (ths) =>
{
this.#p = null;
if (func) func (ths);
return this;
});
}
doHello(a) {console.log("hello: " + a);}
}
class test extends basetest
{
constructor(context)
{
super(context);
this.msg = "test: ";
}
doTest(a) {console.log(this.msg + a);}
}
async function main()
{
(await new test()._then()).doHello("await");//synchronous call
//note, now we use _then instead of then
(new test())._then(a=>{a.doHello("then");a.doTest("then");} );//asynchronous call
}
main();
And suppose a some webgl, with async image loading, generating a mesh and drawing a vertex array object:
class HeightMap extends GlVAObject
{
#vertices = [];
constructor (src, crossOrigin = "")
{
//super(theContextSetup);
let image = new Image();
image.src = src;
image.crossOrigin = crossOrigin;
return new Promise ( (resolve, reject) =>
{
image.addEventListener('load', () =>
{
//reading pixel values from image into this.#vertices
//and generate a heights map
//...
resolve(this);
} );
});
}
///...
}
function drawVao(vao) {/*do something*/}
async function main()
{
let vao = await new HeightMap ("./heightmaps/ArisonaCraterHeightMap.png");
drawVao(vao);
///...
}
/*
//version of main with asynchronous call
async function main()
{
new HeightMap ("./heightmaps/ArisonaCraterHeightMap.png").then (vao => drawVao(vao));
///...
}
*/
main();
Second variant with _then, as it really works in real life:
class HeightMap extends GlVAObject
{
#vertices = [];
constructor (src, crossOrigin = "")
{
//super(theContextSetup);
let image = new Image();
image.src = src;
image.crossOrigin = crossOrigin;
this.#p = Promise ( (resolve, reject) =>
{
image.addEventListener('load', () =>
{
//...
resolve(this);
} );
});
}
async _then (func)
{
return this.#p.then ( (ths) =>
{
this.#p = null;
if (func) func (ths);
return this;
});
}
///...
}
function drawVao(vao) {/*do something*/}
async function main()
{
new HeightMap ("./heightmaps/ArisonaCraterHeightMap.png")._then(vao => drawVao(vao) );
///...
}
/*
//version of main with synchronous call
async function main()
{
vao = (await new HeightMap ("./heightmaps/ArisonaCraterHeightMap.png"))._then();
drawVao(vao);
///...
}*/
main();
You may immediately invoke an anonymous async function that returns message and set it to the message variable. You might want to take a look at immediately invoked function expressions (IEFES), in case you are unfamiliar with this pattern. This will work like a charm.
var message = (async function() { return await grabUID(uid) })()
You should add then function to instance. Promise will recognize it as a thenable object with Promise.resolve automatically
const asyncSymbol = Symbol();
class MyClass {
constructor() {
this.asyncData = null
}
then(resolve, reject) {
return (this[asyncSymbol] = this[asyncSymbol] || new Promise((innerResolve, innerReject) => {
this.asyncData = { a: 1 }
setTimeout(() => innerResolve(this.asyncData), 3000)
})).then(resolve, reject)
}
}
async function wait() {
const asyncData = await new MyClass();
alert('run 3s later')
alert(asyncData.a)
}
#slebetmen's accepted answer explains well why this doesn't work. In addition to the two patterns presented in that answer, another option is to only access your async properties through a custom async getter. The constructor() can then trigger the async creation of the properties, but the getter then checks to see if the property is available before it uses or returns it.
This approach is particularly useful when you want to initialize a global object once on startup, and you want to do it inside a module. Instead of initializing in your index.js and passing the instance in the places that need it, simply require your module wherever the global object is needed.
Usage
const instance = new MyClass();
const prop = await instance.getMyProperty();
Implementation
class MyClass {
constructor() {
this.myProperty = null;
this.myPropertyPromise = this.downloadAsyncStuff();
}
async downloadAsyncStuff() {
// await yourAsyncCall();
this.myProperty = 'async property'; // this would instead by your async call
return this.myProperty;
}
getMyProperty() {
if (this.myProperty) {
return this.myProperty;
} else {
return this.myPropertyPromise;
}
}
}
The closest you can get to an asynchronous constructor is by waiting for it to finish executing if it hasn't already in all of its methods:
class SomeClass {
constructor() {
this.asyncConstructor = (async () => {
// Perform asynchronous operations here
})()
}
async someMethod() {
await this.asyncConstructor
// Perform normal logic here
}
}
The other answers are missing the obvious. Simply call an async function from your constructor:
constructor() {
setContentAsync();
}
async setContentAsync() {
let uid = this.getAttribute('data-uid')
let message = await grabUID(uid)
const shadowRoot = this.attachShadow({mode: 'open'})
shadowRoot.innerHTML = `
<div id="email">A random email message has appeared. ${message}</div>
`
}

reset typescript singleton instance in each unit test

I have a typescript singleton class like so:
export default class MySingleton {
private constructor({
prop1,
prop2,
...
}: MySingletonConfig) {
this.prop1 = prop1 ?? 'defaultProp1';
this.prop2 = prop2;
this.prop3 = prop3 ?? 'defaultProp3';
/* ... some instruction ... */
MySingleton.instance = this;
}
static getInstance(params?: Configuration): MySingleton {
if (!this.instance && !params) {
throw MySingleton.instantiationError;
}
if (!this.instance) {
new MySingleton(params);
return this.instance;
}
return this.instance;
}
}
When I want to unit test it using jest, like so:
describe('getInstance()', () => {
test('it should return the same instance every time', () => {
const params = {
/* ... all the params ... */
};
const mySingleton = MySingleton.getInstance(params);
expect(MySingleton.getInstance()).toEqual(mySingleton);
});
test('it should return the instance with the default value', () => {
const params = {
/* ... ONLY THE REQUIRED PARAMS ... */
};
const mySingleton = MySingleton.getInstance(params);
expect(mySingleton.prop1).toEqual('defaultProp1');
expect(mySingleton.prop3).toEqual('defaultProp3');
});
});
This is failing, because we share the same instance between the 2 tests (as the singleton pattern work), therefore the second instantiation is useless.
Is there a way to reset/destroy the previous instantiation in order to properly check if those default values are properly setted with the second intantiation?
I don't see why you couldn't do:
MySingleton.instance = null;
const mySingleton = MySingleton.getInstance(params);
Ideally the istance property should be really private, but nobody prevents you from adding a reset() method on your class.
It is not particularly neat, as it would basically be for testing purposes only, but at least it would be more close to the canonical implementation of the singleton pattern.
That being said, I would carefully consider if using a singleton is a good idea. It might create a lot of headaches when unit testing your code.
Basically, the same problem you have here could present itself elsewhere when you try to test some code that makes use of your singleton.

Constructor functions in TypeScript, what is missing?

I'm trying to figure out how to use type-safety provided by TypeScript with old plain constructor functions in JS. I have a very simple example, that looks straightforward, but I miss something and can't make it compile with TypeScript:
interface IMyService {
new(): IMyService //I'm not sure if this line should be here, I just trying to make it working...
doSomething(name: string): void
}
function MyService(this: IMyService): void {
let _name = ""
this.doSomething = (name) => {
_name = name
}
}
//The line below won't compile and it saying:
//"new" expression, whose target lacks a construct signature, implicitly has an "any" type
let service = new MyService();
service.setName("Test Name")
What I'm missing? I know the preferred way of using TypeScript is with "class", but in my case I would like to use simple constructor functions.
You cant really type a function declaration (or at least i dont know how). However you can type a variable, and assign a function to it. And then we can define a constructor type:
interface IMyService {
doSomething(name: string): void;
}
interface IMyServiceConstructor {
new(): IMyService;
}
const MyService: IMyServiceConstructor = function(this: IMyService){
//...
};
That can be shortified through using an inline type:
const MyService: { new(): IMyService } = function(){
//...
};
What's stopping you from doing this:
class MyService {
// declare instance method
doSomething: (x: string) => void;
// this is really your function
constructor() {
let _name = "";
this.doSomething = (name) => {
_name = name;
}
}
}
let service = new MyService();
service.doSomething("Test Name");
This emits almost the same code as your original. It's still using a variable local to the constructor function scope, and an instance method instead of a class method. (Instance methods are generally frowned upon because you're creating closures for each instance but that's up to you.)
And TypeScript understands that MyService is newable and all the other goodness you want. Jumping through hoops with constructor type signatures and convincing TypeScript that your function is the right type doesn't seem worth it to me.
Hope that helps.

How to access super class method inside of internal generator function?

Look at this classes, Base and Derived, it's just simple classes that has "name" as property:
class Base {
constructor(name) {
this.name = name;
}
printName() {
console.log("Base: " + this.name);
}
}
class Derieved extends Base {
constructor(name) {
super(name);
}
// Override
printName() {
// IIFE.
(function() {
super.printName(); // Can't use super here
})();
console.log("Derived: " + this.name);
}
}
var obj = new Derieved('John');
obj.printName();
I want to invoke Base::printName from Derieved::printName. But some reason, I have to invoke inside of internal function of Derieved::printName.
But run above code, it fails with:
SyntaxError: 'super' keyword unexpected here
If I saved the reference of parent's method to variable, looks like it can called but can't access any properties, it saids undefined.
TypeError: Cannot read property 'name' of undefined
I just wrote that internal function is just normal function, but actually it's generator function, so I can't use arrow function:
get(match: T, options: IQueryOptions|void): Promise<Array<Object>|Error> {
const superGet = super.get;
return new Promise((resolve, reject) => {
co(function*() {
try {
// I need to invoke parent's get method here!!
const accounts = yield superGet(match, options);
... // do something with accounts
resolve(accounts);
}
catch(err) {
...
}
});
});
}
Is there a way to to this? Why I can't save the reference of parent's method into variable?
super can be accessed only from child method, not from the scope of functions that are called inside this method.
Generator functions are still functions and support binding. This will result in bound function that cannot be identified as generator function by its signature, but as long as coroutine library supports generic iterators (co does), that's fine.
So basically it is
get(...) {
const superGet = super.get;
return new Promise((resolve, reject) => {
co(function*() {
...
const accounts = yield superGet.call(this, match, options);
...
}.bind(this));
});
}
Or even better:
get(...) {
const superGet = super.get.bind(this);
return new Promise((resolve, reject) => {
co(function*() {
...
const accounts = yield superGet(match, options);
...
});
});
}
There are several things that can be improved here. The first one is that it uses promise constructor antipattern. co already returns a promise, there's no need to wrap it with new Promise.
Another thing is that it is beneficial to separate generator method and promisified method, for the sake of seamless inheritance. co supports delegated yields, it makes this even easier:
class Base {
get(...args) {
return co(this._get.bind(this, ...args));
}
* _get(...) { ... }
}
class Child extends Base {
* _get(...) {
...
const accounts = yield* super._get(match, options);
...
}
}
Both TypeScript and Babel support ES2017 async..await and are are able to fall back to co-like generator coroutines in ES6 target output. This makes co efficiently useless in transpiled JS projects, the code above becomes:
class Child extends Base {
async get(...) {
...
const accounts = await super.get(match, options);
...
}
}
Apparently you employ some compiler for the type annotations anyway. In that case, the solution is to just drop the co library and generators, and use proper modern async/await syntax instead, which is supported by TypeScript, Babel and native nodejs:
async get(match: T, options: IQueryOptions|void): Promise<Array<Object>|Error> {
try {
const accounts = await super.get(match, options);
… // do something
return accounts;
}
catch(err) {
…
}
}
super will just work out of the box here.
Arrow functions to the rescue!
class Base {
constructor(name) {
this.name = name;
}
printName() {
console.log("Base: " + this.name);
}
}
class Derieved extends Base {
constructor(name) {
super(name);
}
// Override
printName() {
// IIFE
(() => {
super.printName(); // Can't use super here
})();
console.log("Derived: " + this.name);
}
}
var obj = new Derieved('John');
obj.printName();
basically, the arrow function maintains the this and super contexts, unlike the literal function keyword

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