With JavaScript, when creating a class, while instantiating that class, I want to populate a public property. I can do this using a setter, however, the value I want comes from an external website which I retrieve via an ajax get call. The issue becomes that the new class object does not have the appropriate property value when I create it.
Here's some sample code:
class MyTestClass {
constructor() {
this._ipaddress = "";
}
get ip() {
return this._ipaddress;
}
set ip(value) {
createIpAddr();
}
createIpAddr() {
var myIpAddr = "";
var strUrl = "https://api.ipify.org/?format=json";
$.ajax({
url: strUrl,
success: function(data) {
this._ip = data.ip;
},
//async:false //Performing this in sync mode to make sure I get an IP before the rest of the page loads.
});
return myIpAddr;
}
}
var testobj = new MyTestClass();
console.log(testobj.ip);
The problem here is that I can't be sure the IP will be populated in time to use after creating the new instance of the class. I've tried promises and deffered, but they have the same problem, I can't populate the variable before I need it. I'm trying to adjust the way I am looking at this and adding callbacks, but the issue is that I need the correct value in the class before I can use the class for the next call, where I am passing this object to it.
Is there a simple solution I am over looking? I have been through a million of these threads about async: false, and I don't want to start a new one, but what is a better choice in this case?
I want to set a class property from an ajax response when instantiating the class object.
You could have your constructor return an async IIFE, allowing you to then await the creation of a new class instance.
That would look something like this:
class MyTestClassAsync {
constructor() {
return (async() => {
this._ip = (await this.createIpAddrAsync()).ip;
return this;
})();
}
get ip() {
return this._ip;
}
set ip(value) {
this._ip = value;
}
createIpAddrAsync = () => $.get("https://api.ipify.org/?format=json");
}
async function Main() {
var testobj = await new MyTestClassAsync();
console.log(testobj.ip);
}
Main();
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
I've made the personal decision to append "Async" to the method and class names, just so that it's clear they need to be awaited.
Related
I have a Vue block that I need to bind to a boolean property:
<div class="row" v-if.sync="isThisAllowed">
To calculate that property I need to make an API call using Axios, which has to be asynchronous. I've written the necessary code to get the value:
public async checkAllowed(): Promise<boolean> {
var allowed = false;
await Axios.get(`/api/isItAllowed`).then((result) => {
var myObjects = <MyObject[]>result.data.results;
myObjects.forEach(function (object) {
if (object.isAllowed == true) {
hasValuedGia = true;
}
})
});
return allowed;
}
What I did then - I'm not very experienced with Vue - is to add a property to the Vue model and assign a value to it in created:
public isThisAllowed: boolean = false;
async created() {
this.checkAllowed().then(result => {
this.isThisAllowed = result;
});
}
This works in the sense that the value I'm expecting is assigned to the property. But Vue doesn't like it and complains
Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value.
Most of the values on the model are exposed via getters:
get isSomethingElseAllowed(): boolean {
return this.productCode === ProductTypes.AcmeWidgets;
}
But I need to "await" the value of the async function, which would mean making the getter async which then, of course, makes it a Promise and I can't bind that to my model?
What's the right way to go about this?
You can't define a property that way, instead define isThisAllowed in the data object
as
data: function(){
return {
isThisAllowed: false
}
}
And make checkAllowed into a normal function and set this.isThisAllowed = allowed inside it
I'm trying to make a subclass of an image library on github called Jimp. As far as I can tell from the docs, you don't instantiate the class in the usual way. Instead of saying new Jimp(), it seems the class has a static method called read that acts as a constructor. From the docs...
Jimp.read("./path/to/image.jpg").then(function (image) {
// do stuff with the image
}).catch(function (err) {
// handle an exception
});
It looks like from the docs, that that image returned by read() is an instance allowing the caller to do stuff like image.resize( w, h[, mode] ); and so on.
I'd like to allow my subclass callers to begin with a different static method that reads an image and does a bunch of stuff, summarized as follows...
class MyJimpSubclass extends Jimp {
static makeAnImageAndDoSomeStuff(params) {
let image = null;
// read in a blank image and change it
return Jimp.read("./lib/base.png").then(_image => {
console.log(`image is ${_image}`);
image = _image;
let foo = image.bar(); // PROBLEM!
// ...
// ...
.then(() => image);
}
bar() {
// an instance method I wish to add to the subclass
}
// caller
MyJimpSubclass.makeAnImageAndDoSomeStuff(params).then(image => {
//...
});
You might be able to guess that nodejs gets angry on the line let foo = image.bar();, saying
TypeError image.bar is not a function
.
I think this is understandable, because I got that image using Jimp.read(). Of course that won't return an instance of my subclass.
First idea: Change it to MyJimpSubclass.read(). Same problem.
Second idea: Implement my own static read method. Same problem.
static read(params) {
return super.read(params);
}
Third idea: Ask SO
The implementation of Jimp.read refers to Jimp specifically, so you would have to copy and change it in your subclass (ick, but not going to break anything since the constructor is also part of the API) or make a pull request to have it changed to this and have subclassing explicitly supported:
static read(src) {
return new Promise((resolve, reject) => {
void new this(src, (err, image) => {
if (err) reject(err);
else resolve(image);
});
});
}
Alternatively, you could just implement all your functionality as a set of functions on a module. This would be next on my list after making a pull request. Would not recommend a proxy.
const makeAnImageAndDoSomeStuff = (params) =>
Jimp.read("./lib/base.png").then(image => {
console.log(`image is ${image}`);
let foo = bar(image);
// …
return image;
});
function bar(image) {
// …
}
module.exports = {
makeAnImageAndDoSomeStuff,
bar,
};
Even changing the prototype would be better than a proxy (but this is just a worse version of the first option, reimplementing read):
static read(src) {
return super.read(src)
.then(image => {
Object.setPrototypeOf(image, this.prototype);
return image;
});
}
You have a couple of options. The cleanest is probably to make a subclass like you started, but then implement the Jimp static method on it, as well as your own. In this case, it's not really inheritance, so don't use extends.
class MyJimp {
static read(...args) {
return Jimp.read.apply(Jimp, args);
}
static makeAnImage(params) {
return this.read(params)
.then(image => {
// do stuff
return image
});
}
}
From there, I would make an object which has all of the new functions you want to apply to image:
const JimpImageExtension = {
bar: () => { /* do something */ }
};
Finally, in your static methods, get the image and use Object.assign() to apply your new functions to it:
class MyJimp {
static read(...args) {
return Jimp.read.apply(Jimp, args)
.then(image => Object.assign(image, JimpImageExtension));
}
static makeAnImage(params) {
return this.read(params)
.then(image => {
// do stuff
image.bar();
return image;
});
}
}
This should do the trick by applying your extra functions to the image. You just need to make sure that you apply it at every point that can generate an image (if there is more than just read). Since in the other functions, it's using your version of read(), you only need to add the functions in the one.
Another approach would be if Jimp makes their image class accessible, you could also add them to the prototype of that (though usually in libraries like this, that class is frequently inaccessible or not actually a class at all).
This might be a way to do it. Start with your own read method, and have it change the prototype of the returned object.
static read(...params) {
return super.read(...params).then(image) {
image.prototype = MyJimpSubclass;
resolve(image);
}
}
I currently have the following working code:
Function.prototype.GetLastCallerName = function () {
if (!this.arguments || !this.arguments.callee || !this.arguments.callee.caller) return null;
var result = /^function\s+([\w\$]+)\s*\(/.exec(this.arguments.callee.caller.toString());
this.LastCaller = result ? result[1] : 'Anonymous';
return this.LastCaller;
};
I picked up that code from another thread. As you can see, it extends the Function.prototype in order to add a method called GetLastCallerName, which picks the last calling function name and (1) sets it to LastCaller on Function.LastCaller and (2) returns it.
In order to make it work:
function MyFunction1() {
MyFunction1.GetLastCallerName();
console.log(MyFunction.LastCaller);
}
function MyFunction2() {
MyFunction1();
}
MyFunction2();
What I'd like to be able to do: Eliminate the need to use GetLastCallerName() every time and extend Function in order to perform that get every time any function is called.
I'm struggling to follow what you have tried so far with your example, but I think I get the idea of what you'd like to do. Why not leverage classes, and extend on them for your use case. Check out the following example...
class Base {
baseFn() {
console.log('from base');
}
}
class Thing extends Base {
fn1() {
this.baseFn();
}
}
let thingee = new Thing();
thingee.fn1();
So baseFn is now always called when fn1 is called.
JSFiddle Link - class demo
In some of your comments it looks like you are wanting to get the "last calling function's name." How about passing back the instance of the caller itself to the parent? This would surely give you even more flexibility because now you can sculpt your caller however you wish. Check out the following...
class Base {
baseFn(caller) {
console.log(caller.id); // 1
}
}
class Thing extends Base {
constructor(id) {
super();
this.id = id;
}
fn1() {
this.baseFn(this);
}
}
let thingee = new Thing('1');
thingee.fn1();
Now you can add whatever you'd like to your Thing instance, in this case, an object with an id of 1 which can be inspected when fn1 propagates up to baseFn
JSFiddle Link - caller demo
I have a config.json that I am going to load into my app as a Backbone Model like:
var Config = Backbone.Model.extend({
defaults: {
base: ''
},
url: 'config.json'
});
Other models should be dependent on some data contained in Config like:
var ModelA = Backbone.Collection.extend({
initialize: function(){
//this.url should be set to Config.base + '/someEndpoint';
}
});
In above example, ModelA's url property is dependent on Config's base property's value.
How do I go about setting this up properly in a Backbone app?
As I see it, your basic questions are:
How will we get an instance of the configuration model?
How will we use the configuration model to set the dependent model's url?
How can we make sure we don't use the url function on the dependent model too early?
There are a lot of ways to handle this, but I'm going to suggest some specifics so that I can just provide guidance and code and "get it done," so to speak.
I think the best way to handle the first problem is to make that configuration model a singleton. I'm going to provide code from backbone-singleton GitHub page below, but I don't want the answer to be vertically long until I'm done with the explanation, so read on...
var MakeBackboneSingleton = function (BackboneClass, options) { ... }
Next, we make a singleton AppConfiguration as well as a deferred property taking advantage of jQuery. The result of fetch will provide always(callback), done(callback), etc.
var AppConfiguration = MakeBackboneSingleton(Backbone.Model.extend({
defaults: {
base: null
},
initialize: function() {
this.deferred = this.fetch();
},
url: function() {
return 'config.json'
}
}));
Now, time to define the dependent model DependentModel which looks like yours. It will call AppConfiguration() to get the instance.
Note that because of MakeBackboneSingleton the follow is all true:
var instance1 = AppConfiguration();
var instance2 = new AppConfiguration();
instance1 === instance2; // true
instance1 === AppConfiguration() // true
The model will automatically fetch when provided an id but only after we have completed the AppConfiguration's fetch. Note that you can use always, then, done, etc.
var DependentModel = Backbone.Model.extend({
initialize: function() {
AppConfiguration().deferred.then(function() {
if (this.id)
this.fetch();
});
},
url: function() {
return AppConfiguration().get('base') + '/someEndpoint';
}
});
Now finally, putting it all together, you can instantiate some models.
var newModel = new DependentModel(); // no id => no fetch
var existingModel = new DependentModel({id: 15}); // id => fetch AFTER we have an AppConfiguration
The second one will auto-fetch as long as the AppConfiguration's fetch was successful.
Here's MakeBackboneSingleton for you (again from the GitHub repository):
var MakeBackboneSingleton = function (BackboneClass, options) {
options || (options = {});
// Helper to check for arguments. Throws an error if passed in.
var checkArguments = function (args) {
if (args.length) {
throw new Error('cannot pass arguments into an already instantiated singleton');
}
};
// Wrapper around the class. Allows us to call new without generating an error.
var WrappedClass = function() {
if (!BackboneClass.instance) {
// Proxy class that allows us to pass through all arguments on singleton instantiation.
var F = function (args) {
return BackboneClass.apply(this, args);
};
// Extend the given Backbone class with a function that sets the instance for future use.
BackboneClass = BackboneClass.extend({
__setInstance: function () {
BackboneClass.instance = this;
}
});
// Connect the proxy class to its counterpart class.
F.prototype = BackboneClass.prototype;
// Instantiate the proxy, passing through any arguments, then store the instance.
(new F(arguments.length ? arguments : options.arguments)).__setInstance();
}
else {
// Make sure we're not trying to instantiate it with arguments again.
checkArguments(arguments);
}
return BackboneClass.instance;
};
// Immediately instantiate the class.
if (options.instantiate) {
var instance = WrappedClass.apply(WrappedClass, options.arguments);
// Return the instantiated class wrapped in a function so we can call it with new without generating an error.
return function () {
checkArguments(arguments);
return instance;
};
}
else {
return WrappedClass;
}
};
I have a simple requirement, I need add the same code to hundreds of other JavaScript functions, the code can be executed at the end of the function, is there a handy way of doing it, like attach an function to another function dynamically, I think yes, because JavaScript is so powerful and too powerful, any ideas?
Note, I need dynamically assign new code or function to existing functions without change existing function's code, please give a solid solution, I can do it in hacky way, but no hacky way please!
The first method that comes to mind is simply create another function:
function primaryFunction() {
// ...
utilityMethod();
}
function otherPrimaryFunction() {
// ...
utilityMethod();
}
function utilityMethod() { ... }
Now utilityMethod() gets called from the end of each other primary function.
There's also a method which requires more code refactoring but is better in the long term: classes/prototypes.
Essentially, you have one "constructor" function which takes a number of parameters for the "class" and returns an class-like object:
function constructor(someClassField, anotherField) {
this.aField = someClassField;
this.fieldTwo = anotherField;
return this;
}
Now if you call this and pass some parameters, you get a class out:
var myClass = new constructor("1", "2");
myClass.aField == "1";
myClass.fieldTwo == "2";
So: If you define your utility method as above, then you can use this: for every primary function you instantiate a new instance of the constructor, with the final code looking like this:
function constructor(primaryFunction) {
this.function = primaryFunction;
this.call = function() {
this.function();
utilityMethod();
}
this.call();
return this;
}
function utilityMethod() { ... }
var primaryMethod = new constructor(function() { ... });
The creation of primaryMethod now automatically calls the primary function followed by the utility method, before returning the object so you can re-call both if you want to.