class Foo{
}
var instance: Foo;
export function getFooInstance(){
/* logic */
}
or
export class Foo{
private static _instance;
private constructor(){};
public getInstance(){/* logic */}
}
// Use it like this
Foo.getInstance()
I would like to ensure that only one instance of the object is created. Does anyone have any other suggestions besides this method?
Check out the Typescript Playground here.
In order to utilize the getter within the class, it must be declared static:
export class Bar{
private static _instance;
private constructor(){};
public static getInstance(){/* implementation */}
}
It's worth noting that although the compiler enforces private visibility, in runtime it is still feasible to circumvent it, even inadvertently, such as when accessed directly from Javascript.
To ensure complete concealment, you can employ a module or namespace:
Module approach:
export interface IBar {}
class Bar implements IBar {}
var instance: Bar;
export function getBarInstance(): IBar {
/* logic */
return instance;
}
This code snippet introduces the IBar interface (which is also exported), ensuring that whoever obtains an instance is aware of the interface but not the actual class.
Namespace approach:
namespace Bar {
export interface IBar {}
class BarClass implements IBar {}
const instance = new BarClass();
export function getInstance(): IBar {
return instance;
}
}
When working with JS and TypeScript, if you are looking to have just one instance, consider enforcing it within the language itself by exporting an object literal.
const Singleton = {
doSomething() {
}
}
export default Singleton;
In my opinion, this adheres to the principle of KISS (Keep It Simple, Stupid), involves minimal boilerplate, and prevents the creation of multiple instances.
Alternatively, you can directly export functions as well. Remember that a module can act as a singleton for you.
export function doSomething() {
}
If you want to treat the imported module as an object, you can use the import *. However, I recommend the first approach if the functions truly belong to the object and are not all stateless static functions.
import * as Singleton from './Singleton';
Singleton.doSomething();
When deciding whether to allow the creation of a new instance for a singleton class, one must consider various factors. In some cases, the getInstance method can be excluded and the class constructor itself can function as a singleton factory:
class Foo {
private static _instance;
constructor(...args) {
if (Foo._instance) {
return Foo._instance;
}
// perform additional operations here if needed
Foo._instance = this;
};
}
A similar approach can also be applied using decorators for different classes, such as:
@singleton
class Foo { ... }
However, due to certain typing issues with TypeScript decorators as described here, it is recommended to implement custom inheritance logic within the singleton decorator instead of relying on Singleton extends Class:
function singleton(Class) {
function extend(sub, sup) {
for (var prop in sup)
if (sup.hasOwnProperty(prop))
sub[prop] = sup[prop];
function __() {
this.constructor = sub;
}
__.prototype = sup.prototype;
sub.prototype = new __();
};
const Singleton = <any>function (...args) {
if (Singleton._instance) {
return Singleton._instance;
}
Class.apply(this, args);
Singleton._instance = this;
}
extend(Singleton, Class);
return Singleton;
}
Although this approach may impact typing, it helps maintain a clean syntax.
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