The problem with `Set` is that its `add` method returns `this` instead of
`boolean`. If `Set.prototype.add` returned `boolean`, I would have used `Set`.
That’s why in the `select` method of my sample code, I use a custom defined
method named `pushIfAbsent`. The actual type of `_values` is not `Array` but a
subclass of `Array`.
``` js
export class MyArray<E> extends Array<E> {
/**
* Adds [item] to the end of this array if it's not already in this array.
*
* Returns `true` is [item] was added, `false` otherwise.
*/
pushIfAbsent(item: E): boolean {
if (!this.includes(item)) {
this.push(item);
return true;
}
return false;
}
/**
* Removes the first occurrence of [item] from this array.
*
* Returns `true` if [item] was in this array, `false` otherwise.
*/
remove(item: E): boolean {
const i = this.indexOf(item);
if (i >= 0) {
this.splice(i, 1);
return true;
}
return false;
}
}
```
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