The author of this article uses singletons for the service layer in this example Node api:
https://html5hive.org/how-to-create-rest-api-with-node-js-and-express/
He states, "We only want there to ever be one instance of our player service, so instead of exporting the class itself, we’ll export a new instance of it. Module files in node are only ever executed once, so this effectively gives us a singleton."
'use strict';
var uuid = require('node-uuid');
class PlayersService {
constructor() {
this.players = [];
}
getPlayers() {
return this.players;
}
getSinglePlayer(playerId) {
var player = this.players.filter(p => p.id === playerId)[0];
return player || null;
}
addPlayer(info) {
// prevent a bit of bad/duplicate data
if (!info || this.players.filter(p => (p.firstName === info.firstName && p.lastName === info.lastName)).length > 0) {
return false;
}
info.id = uuid.v4();
this.players.push(info);
return true;
}
updatePlayer(playerId, info) {
var player = this.getSinglePlayer(playerId);
if (player) {
player.firstName = info.firstName ? info.firstName : player.firstName;
player.lastName = info.lastName ? info.lastName : player.lastName;
player.displayName = info.displayName ? info.displayName : player.displayName;
return true;
}
return false;
}
}
module.exports = new PlayersService();
Which seems reasonable since the function of these services is to provide the same implementation for the controllers that use them.
However, in this post:
On Design Patterns: When to use the Singleton?
the poster asks for a legitimate use case for singletons other than a Logger class. Several people responded to his question by saying that singletons should never be used.
But isn't the use of singletons for services like the one I've copied here a legitimate use case and a best practice so that you are not creating multiple instances that provide the same implementation? Thanks.
Related
I have NodeJS program.
In one class, I have various utility methods. One function, safeGithubPush, calls safeString, another func in the same class
module.exports = {
safeString(stringToCheck) {
console.log(validator.isAscii(stringToCheck), validator.matches(stringToCheck, /^((\w)*[-.]?(\w)*)*$/))
return (
validator.isAscii(stringToCheck) &&
validator.matches(stringToCheck, /^((\w)*[-.]?(\w)*)*$/)
);
},
safeGithubPush(currentJob) {
if (
!currentJob ||
!currentJob.payload ||
!currentJob.payload.repoName ||
!currentJob.payload.repoOwner ||
!currentJob.payload.branchName
) {
this.logIn(
currentJob,
`${' (sanitize)'.padEnd(15)}failed due to insufficient job definition`
);
throw invalidJobDef;
}
if (
this.safeString(currentJob.payload.repoName) &&
this.safeString(currentJob.payload.repoOwner) &&
this.safeString(currentJob.payload.branchName)
) {
return true;
}
throw invalidJobDef;
},
}
While this.logIn(), another func in the utility class, works just fine, I get the error for safeString:
Error caught by first catch: TypeError: this.safeString is not a function
I followed a solution offer by another SO post:
safeString: function(stringToCheck){
...
}
safeGithubPush(currentJob) {
...
if (
this.safeString(currentJob.payload.repoName) &&
this.safeString(currentJob.payload.repoOwner) &&
this.safeString(currentJob.payload.branchName)
) {
return true;
}
}
But this also gets a, TypeError: this.safeString is not a function.
I'm not using arrow functions, which is the explanation for this error on a different SO post
I don't think the reason is determinable with the code you are currently presenting. It likely has something to do with how you are calling safeGithubPush. If you do something that would change the this binding the this.safeString is going to fail.
const foo = {
fizz() {
console.log("fizz");
},
buzz() {
this.fizz();
}
};
// "this" is correct
foo.buzz();
// "this" has no fizz to call
const myFizz = foo.buzz;
myFizz();
Considering you are attaching these to module.exports I am going to guess that you pull these functions off in a require call and then try to use them bare which makes the problem obvious after looking at my example above:
// Ignore these 2 lines, they let this look like node
const module = {};
const require = () => module.exports;
// Ignore above 2 lines, they let this look like node
// Your module "some.js"
module.exports = {
safeString(str) {
return true;
},
safeGithubPush(currentJob) {
if (!this.safeString("some")) {
throw new Error("Not safe");
}
return true;
}
};
try {
// Some consumer module that doesn't work
const {safeGithubPush} = require("./some.js");
const isItSafe = safeGithubPush();
console.log(`Safe? ${isItSafe}`);
} catch (err) {
console.error("Didn't bind right \"this\"");
}
try {
// Some consumer module that DOES work
const someModule = require("./some.js");
const isItSafe = someModule.safeGithubPush();
console.log(`Safe? ${isItSafe}`);
} catch (err) {
console.error(err);
}
I would restructure this code. You say these are utility functions which makes me think you don't really want to have to structure them with this in mind.
Instead of attaching them all to module.exports at their definition, define them outside and directly reference the functions you want to use, then attach them to exports so other modules can use the functions:
function safeString(stringToCheck) {
return true;
}
function safeGithubPush(currentJob) {
if (!safeString("some")) {
throw new Error("Not safe");
}
return true;
}
module.exports = {
safeString,
safeGithubPush
};
I am making a simple note taking app to learn node and ES6. I have 3 modules - App, NotesManager and Note. I am importing the Note class into the NotesManager and am trying to instantiate it in its addNote function. The problem is that even though the import is correct, it turns out to be undefined inside the class definition. A simpler solution would be to just instantiate the NotesManager class and add the Note class to its constructor however, I want to have NotesManager as a static utility class.
Here is my code.
Note.js
class Note {
constructor(title, body) {
this.title = title;
this.body = body;
}
}
module.exports = Note;
NotesManager.js
const note = require("./Note");
console.log("Note: ", note); //shows correctly
class NotesManager {
constructor() {}
static addNote(title, body) {
const note = new note(title, body); //Fails here as note is undefined
NotesManager.notes.push(note);
}
static getNote(title) {
if (title) {
console.log(`Getting Note: ${title}`);
} else {
console.log("Please provide a legit title");
}
}
static removeNote(title) {
if (title) {
console.log(`Removing Note: ${title}`);
} else {
console.log("Please provide a legit title");
}
}
static getAll() {
//console.log("Getting all notes ", NotesManager.notes, note);
}
}
NotesManager.notes = []; //Want notes to be a static variable
module.exports.NotesManager = NotesManager;
App.js
console.log("Starting App");
const fs = require("fs"),
_ = require("lodash"),
yargs = require("yargs"),
{ NotesManager } = require("./NotesManager");
console.log(NotesManager.getAll()); //works
const command = process.argv[2],
argv = yargs.argv;
console.log(argv);
switch (command) {
case "add":
const title = argv.title || "No title given";
const body = argv.body || "";
NotesManager.addNote(title, body); //Fails here
break;
case "list":
NotesManager.getAll();
break;
case "remove":
NotesManager.removeNote(argv.title);
break;
case "read":
NotesManager.getNote(argv.title);
break;
default:
notes.getAll();
break;
}
Is it possible for me to create a strict utility class which I can use without instantiating like in Java? Pretty new here and have tried searching for it without any luck. Thank you for your help.
When you do this:
const note = new note(title, body);
you redefine note shadowing the original note from the outer scope. You need to pick a different variable name.
Something like this should work better:
static addNote(title, body) {
const some_note = new note(title, body); //Fails here as note is undefined
NotesManager.notes.push(some_note);
}
I have a function that I need to pass to a class I have defined in nodeJs.
The use case scenario is I want to give the implementer of the class the control of what to do with the data received from createCall function. I don't mind if the method becomes a member function of the class. Any help would be appreciated.
//Function to pass. Defined by the person using the class in their project.
var someFunction = function(data){
console.log(data)
}
//And I have a class i.e. the library.
class A {
constructor(user, handler) {
this.user = user;
this.notificationHandler = handler;
}
createCall(){
var result = new Promise (function(resolve,reject) {
resolve(callApi());
});
//doesn't work. Keeps saying notificationHandler is not a function
result.then(function(resp) {
this.notificationHandler(resp);
}) ;
//I want to pass this resp back to the function I had passed in the
// constructor.
//How do I achieve this.
}
callApi(){ ...somecode... }
}
// The user creates an object of the class like this
var obj = new A("abc#gmail.com", someFunction);
obj.createCall(); // This call should execute the logic inside someFunction after the resp is received.
Arrow functions (if your Node version supports them) are convenient here:
class A {
constructor(user, handler) {
this.user = user;
this.notificationHandler = handler;
}
createCall() {
var result = new Promise(resolve => {
// we're fine here, `this` is the current A instance
resolve(this.callApi());
});
result.then(resp => {
this.notificationHandler(resp);
});
}
callApi() {
// Some code here...
}
}
Inside arrow functions, this refers to the context that defined such functions, in our case the current instance of A. The old school way (ECMA 5) would be:
createCall() {
// save current instance in a variable for further use
// inside callback functions
var self = this;
var result = new Promise(function(resolve) {
// here `this` is completely irrelevant;
// we need to use `self`
resolve(self.callApi());
});
result.then(function(resp) {
self.notificationHandler(resp);
});
}
Check here for details: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions#No_separate_this
I've tried to search for instance caching and singletons on Google and StackOverflow without success, seeing only posts about module.exports, if you know a post that answers this question, feel free to reference it. Thank you!
I have an application that needs to work on a set of objects that rarely change, and hence need to be cached for performance optimisation.
Here is a toy example where a single property is set directly.
When I call the application, I export an object that will contain the set of cached objects in assets_cached.js:
const Assets = {};
module.exports.Assets = Assets;
In another module of the application I have an ES6 class:
const _ = require('lodash')
const { Assets } = require('./assets_cached')
class Asset {
constructor(id, some_property) {
if (id in Assets) {
// Update instance data with cached properties
_.assign(this, Assets_cached[id]);
} else {
// If it's not cached, create a new object
this.id = id;
this.some_property = some_property;
// Cache this object
Assets_cached[id] = this;
}
}
getProperty() {
return this.some_property;
}
setProperty(value) {
this.some_property = value;
// Is there a way of avoiding having to do this double assignment?
Assets_cached[id].some_property = value;
}
}
module.exports = Asset;
How may I avoid having to set the some_property twice (in the current instance and the cache, while ensuring that other instances are updated in parallel)?
Ideally I'd like to do something like:
if (id in Assets) {
this = Assets.cached[id]
}
inside the constructor, but this is not possible.
What's the most elegant and correct way of making this work?
Ideally I'd like to do something like this = Assets.cached[id] inside the constructor
The magic keyword here is return. You can just return an arbitrary object from the constructor and it will be used instead of this.
constructor(id, some_property) {
if (id in Assets) {
// use cached instance instead of creating a new one
return Assets_cached[id];
} else {
this.id = id;
this.some_property = some_property;
// Cache this object
Assets_cached[id] = this;
}
}
Here is the approach to the comment that was made some half an hour ago ...
const { Assets_cached } = require('./assets_cached');
// const { AssetStore } = require('./assetstore');
class Asset {
constructor(id, some_property) { // clean/lean constructor.
this.id = id;
this.some_property = some_property;
}
getProperty() {
return this.some_property;
}
setProperty(value) {
this.some_property = value;
}
}
function isAsset(type) {
// poor man's approach ... change to something more feasible.
return (type instanceof Asset);
}
function createAsset(id, some_property) { // factory that also handles caching.
var
asset = Assets_cached[id];
// asset = AssetStore.get(id);
if (!(asset && isAsset(asset))) {
asset = Assets_cached[id] = (new Asset(id, some_property));
// AssetStore.put(id, (asset = new Asset(id, some_property)));
}
return asset;
}
module.exports = {
create : createAsset,
isAsset : isAsset
};
Note
One also should consider providing a minimal API to Assets_cached, something like put/set, get and delete instead of Assets_cached being an entirely exposed, plain key-value store.
I am getting this error in my code
TypeError: account.on() is not a function
Where did i go wrong?
Code
var events = require('events');
function Account() {
this.balance = 0;
events.EventEmitter.call(this);
this.deposit = function(amount) {
this.balance += amount;
this.emit('balanceChanged');
};
this.withdraw = function(amount) {
this.balance -= amount;
this.emit('balanceChanged');
};
}
Account.prototype._proto_ = events.EventEmitter.prototype;
function displayBalance() {
console.log('Account balance : $%d', this.balance);
}
function checkOverdraw() {
if (this.balance < 0) {
console.log('Account overdrawn!!!');
}
}
function checkgoal(acc, goal) {
if (acc.balance > goal) {
console.log('Goal Achieved!!!');
}
}
var account = new Account();
account.on('balanceChanged', displayBalance);
account.on('balanceChanged', checkOverdraw);
account.on('balanceChanged', function() {
checkgoal(this, 1000);
});
account.deposit(220);
account.deposit(320);
account.deposit(600);
account.withdraw(1200);
Your example code is not idiomatic Node JS.
I'd strongly recommend you follow the recommended best practices when creating new inheritable objects, as in:
var util=require('util');
var EventEmitter = require('events').EventEmitter;
var Account = function(){
EventEmitter.call(this); // should be first
this.balance=0; // instance var
};
util.inherits(Account,EventEmitter);
Account.prototype.deposit = function(amount){
this.balance += amount;
this.emit('balanceChanged');
};
Account.prototype.withdraw = function(amount){
this.balance -= amount;
this.emit('balanceChanged');
};
var account = new Account();
var displayBalance = function(){
console.log("Account balance : $%d", this.balance);
};
account.on('balanceChanged',displayBalance);
account.deposit(200);
account.withdraw(40);
// ... etc. ....
Which, when run displays:
Account balance : $200
Account balance : $160
Best practices are there so that
your code can be expressed in a way that is easy for others to understand
you don't run into unexpected problems when you try to replicate functionality that is already defined, possibly complex and difficult to understand.
The reason that util.inherits exists is so you don't have to worry about how the prototype chain is constructed. By constructing it yourself, you will often run into the problem you experienced.
Also, since the current Node runtime (>6.0) also includes most of the ES6 spec, you can also (and really should) write your code as:
const util = require('util');
const EventEmitter = require('events').EventEmitter;
const Account = () => {
EventEmitter.call(this);
this.balance = 0;
};
util.inherits(Account,EventEmitter);
Account.prototype.deposit = (val) => {
this.balance += val;
this.emit('balanceChanged');
};
Account.prototype.withdraw = (val) => {
this.balance -= val;
this.emit('balanceChanged');
};
The use of the const keyword assures the variables you create cannot be changed inadvertently or unexpectedly.
And the use of the "fat arrow" function definition idiom (() => {}) is more succinct and thus quicker to type, but also carries the added benefit that it preserves the value of this from the surrounding context so you never have to write something like:
Account.prototype.doSomething = function() {
var self = this;
doSomething(val, function(err,res){
if(err) {
throw err;
}
self.result=res;
});
};
which, using the 'fat arrow' construct becomes:
Account.prototype.doSomething = () => {
doSomething(val, (err,res) => {
if(err) {
throw err;
}
this.result=res; // where 'this' is the instance of Account
});
};
The "fat arrow" idiom also allows you to do some things more succinctly like:
// return the result of a single operation
const add = (a,b) => a + b;
// return a single result object
const getSum = (a,b) => {{a:a,b:b,sum:a+b}};
Another way to create inheritable "classes" in ES6 is to use its class construction notation:
const EventEmitter = require('events');
class Account extends EventEmitter {
constructor() {
super();
this._balance = 0; // start instance vars with an underscore
}
get balance() { // and add a getter
return this._balance;
}
deposit(amount) {
this._balance += amount;
this.emit('balanceChanged');
}
withdraw(amount) {
this._balance -= amount;
this.emit('balanceChanged');
}
}
It should be noted that both ways of constructing inheritable prototypal objects is really the same, except that the new class construction idiom adds syntactic "sugar" to bring the declaration more in-line with other languages that support more classical object orientation.
The ES6 extensions to node offer many other benefits worthy of study.