unionBy

/**
 * @param {(value: unknown) => unknown} iteratee The iteratee invoked per element.
 * @param {...Array<unknown>} arrays The arrays to inspect.
 * @returns {Array<unknown>} Returns the new array of combined values.
 */
export default function unionBy(iteratee, ...arrays) {
  // Result array that stores unique values in insertion order.
  const result = [];

  // Track computed uniqueness values.
  // Example:
  // Math.floor(2.1) => 2
  // Math.floor(2.3) => 2
  // So only the first one should be kept.
  const seen = new Set();

  // Iterate through every array provided.
  for (const array of arrays) {
    // Skip invalid/falsy arrays safely.
    if (!Array.isArray(array)) {
      continue;
    }

    // Process each value in the current array.
    for (const value of array) {
      // Compute uniqueness criterion using iteratee.
      const computed = iteratee(value);

      // Only add value if we haven't seen
      // the computed key before.
      if (!seen.has(computed)) {
        seen.add(computed);
        result.push(value);
      }
    }
  }

  return result;
}

/**
unionBy((value: any) => value, [2], [1, 2]); // => [2, 1]

unionBy(Math.floor, [2.1], [1.2, 2.3]); // => [2.1, 1.2]

unionBy((o) => o.x, [{ x: 1 }], [{ x: 2 }, { x: 1 }]); // => [{ 'x': 1 }, { 'x': 2 }]
 */