112 lines
2.9 KiB
Markdown
112 lines
2.9 KiB
Markdown
---
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tags:
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- OOP
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- design-patterns
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---
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# Singleton pattern
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As the name suggests, the Singleton pattern is best used in scenarios where you
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only want to use a single, unique instance of a class. This is for functionality
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that should only be executed once in the lifecycle of an application. The
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canonical case would be some kind of bootstrapping process required at
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initialisation, or initial configuration for an app.
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## Use-cases
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- Establishing connection to backend database
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- API connection (managing tokens so that you are not sending multiple
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unnecessary calls)
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- Configuration settings for an app
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- Global state stores in Redux and the React Context API
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> With this pattern only one instance of the class can exist. If no instance of
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> the singleton class exists then a new instance is created and returned, but if
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> an instance already exists, then the reference to the existing instance is
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> returned. A singleton should be immutable by the consuming code, and there
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> should be no danger of instantiating more than one of them.
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## Implementation
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This pattern can be implemented either with a object or a class.
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In the example below, a global store for a modern JS application is created
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using an object.
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### Using an object
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We ensure singularity by:
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- using `const` for the object so that it cannot be reassigned
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- using `Object.freeze` so that the fields cannot be overwritten, deleted, or
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additional fields added
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```js
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const _data = [];
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// Methods to add and retrieve data from the store
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const UserStore = {
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add: (item) => _data.push(item),
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get: (id) => _data.find((d) => d.id === id),
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};
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Object.freeze(UserStore);
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export default UserStore;
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```
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### Using a class
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The same process can be created using classes:
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```js
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class UserStore {
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constructor() {
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this._data = [];
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}
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add(item) {
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this._data.push(item);
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}
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get(id) {
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return this._data.find((d) => d.id === id);
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}
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}
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const instance = new UserStore();
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Object.freeze(instance);
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export default instance;
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```
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Bear in mind that this is really just an ES6 module with `freeze` applied. The
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only real difference between a singleton and a module is that there should only
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be one instance of it.
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With classes, there is a further step that can be taken to prevent more than
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once instance of the class being created. This is necessary if you want to be
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really strict because it is of course possible for a well motivated person to
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get around the safeguards we established above. We do this by having the class
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itself check whether there is already an instance of itself. If so, it just
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returns the already existing instance. We do this by using the self-reference
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inherent in `this` :
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```jsx
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class UserStore {
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constructor() {
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if (!UserStore.instance) {
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this._data = [];
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UserStore.instance = this;
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}
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return UserStore.instance;
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}
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//rest is the same code as preceding example
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}
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const instance = new UserStore();
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Object.freeze(instance);
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export default instance;
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```
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