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# Design Patterns
## Singletons
When exactly one object is needed for a given task, prefer to define it as a
`class` rather than as an object literal. Prefer also to explicitly restrict
instantiation, unless flexibility is important (e.g. for testing).
```javascript
// bad
const MyThing = {
prop1: 'hello',
method1: () => {}
};
export default MyThing;
// good
class MyThing {
constructor() {
this.prop1 = 'hello';
}
method1() {}
}
export default new MyThing();
// best
export default class MyThing {
constructor() {
if (!this.prototype.singleton) {
this.init();
this.prototype.singleton = this;
}
return this.prototype.singleton;
}
init() {
this.prop1 = 'hello';
}
method1() {}
}
```
## Manipulating the DOM in a JS Class
When writing a class that needs to manipulate the DOM guarantee a container option is provided.
This is useful when we need that class to be instantiated more than once in the same page.
Bad:
```javascript
class Foo {
constructor() {
document.querySelector('.bar');
}
}
new Foo();
```
Good:
```javascript
class Foo {
constructor(opts) {
document.querySelector(`${opts.container} .bar`);
}
}
new Foo({ container: '.my-element' });
```
You can find an example of the above in this [class][container-class-example];
[container-class-example]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/mini_pipeline_graph_dropdown.js
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