For Two Sum, walk the array once and remember each value's index in a Map. For each number, check whether its complement has already been seen:

/** Returns the indexes of two different elements that add up to target, or null. */
export function twoSum(nums: readonly number[], target: number): [number, number] | null {
  const seen = new Map<number, number>();
  for (let i = 0; i < nums.length; i++) {
    const partner = seen.get(target - nums[i]);
    if (partner !== undefined) return [partner, i];
    seen.set(nums[i], i);
  }
  return null;
}

/** Removes duplicates, keeping the first occurrence of each value in order. */
export const dedupe = <T>(items: readonly T[]): T[] => [...new Set(items)];

Both are O(n) in time and space. Set keeps insertion order, so dedupe preserves the original order. It compares values with SameValueZero: NaN counts as equal to itself, but two different objects with the same content don't, which is worth saying before you're asked.

The tests are where you show testing judgment:

import { describe, expect, it } from 'vitest';
import { dedupe, twoSum } from './arrays';

describe('twoSum', () => {
  it('finds a pair', () => expect(twoSum([2, 7, 11, 15], 9)).toEqual([0, 1]));
  it('does not use the same element twice', () => expect(twoSum([3, 5], 6)).toBeNull());
  it('uses two equal values at different indexes', () => expect(twoSum([3, 3], 6)).toEqual([0, 1]));
  it('handles negatives and zero', () => expect(twoSum([-4, 0, 4], 0)).toEqual([0, 2]));
  it('returns null when there is no pair or no input', () => {
    expect(twoSum([1, 2, 3], 100)).toBeNull();
    expect(twoSum([], 0)).toBeNull();
  });
});

describe('dedupe', () => {
  it('keeps the first occurrence, in order', () => expect(dedupe([3, 1, 3, 2, 1])).toEqual([3, 1, 2]));
  it('treats NaN as one value', () => expect(dedupe([NaN, NaN])).toEqual([NaN]));
  it('keeps objects that are equal in content but not identical', () => expect(dedupe([{ a: 1 }, { a: 1 }])).toHaveLength(2));
});

Before coding, ask what the function should return when there is no pair, or when several pairs qualify. Clarifying the contract is part of what the interviewer is assessing.

Sources: MDN: Map, MDN: Set