retain.ts

import type { PrototypeStruct } from '../index.js';

type RetainPredicate<T> = (element: T, index: number) => void;
interface Retain<T> {
  retain(predicate: RetainPredicate<T>): void;
}

export const retain: PrototypeStruct = {
  label: 'retain',
  fn: function arrayRetain<T = unknown>(predicate: RetainPredicate<T>): void {
    const ctx = this as unknown as T[];

    if ('function' !== typeof(predicate)) {
      throw new TypeError(`Expect 'predicate' to be a function`);
    }

    const len = ctx.length;

    if (!len) return;

    const retained = ctx.filter(predicate);

    ctx.length = 0;
    for (const n of retained) ctx.push(n);
  },
};

declare global {
  interface Array<T> extends Retain<T> {}
}