filterMap

The filterMap utility maps and filters an array in a single pass. Return undefined from the callback to skip an item.

Source Code

View Source Code

import { assert } from '../function/assert';
import { IS_ARRAY_ERROR_MSG, isArray } from '../typed/isArray';

/**
 * Maps and filters in one pass. Returning `undefined` drops an item.
 */
export function filterMap<T, R>(array: T[], callback: (item: T, index: number, array: T[]) => R | undefined): R[] {
  assert(isArray(array), IS_ARRAY_ERROR_MSG, { args: { array }, type: TypeError });

  const result: R[] = [];

  for (let index = 0; index < array.length; index++) {
    const value = callback(array[index], index, array);

    if (value !== undefined) {
      result.push(value);
    }
  }

  return result;
}

Features

• Isomorphic: Works in both Browser and Node.js.
• Single Pass: Map and filter in one iteration for readability and performance.
• Explicit Filtering: Return undefined to drop an item.

API

function filterMap<T, R>(array: T[], callback: (item: T, index: number, array: T[]) => R | undefined): R[];

Parameters

• array: The input array.
• callback: Transformation applied to each element. Return undefined to drop an item.

Returns

A new array containing the mapped values of elements where the callback returned a value.

Examples

Synchronous Selection

import { filterMap } from '@vielzeug/toolkit';

const numbers = [10, 20, 30, 40];

// Double only the numbers greater than 20
const result = filterMap(numbers, (x) => (x > 20 ? x * 2 : undefined));
// [60, 80]

Asynchronous Selection

WARNING

filterMap does not support async callbacks. Use parallel or Promise.all for async mapping:

import { parallel } from '@vielzeug/toolkit';

const ids = [1, 2, 3];
const details = await parallel(ids, async (id) => fetchUser(id), { limit: 3 });

Default Filtering

import { filterMap } from '@vielzeug/toolkit';

const data = [1, null, 2, undefined, 3];

// Return undefined to skip nullish values
const doubled = filterMap(data, (x) => (x == null ? undefined : x * 2));
// [2, 4, 6]

Implementation Notes

• Throws TypeError if the first argument is not an array.
• The callback controls both mapping and filtering.
• undefined callback results are omitted from the output.

See Also

• toggle: Add or remove an item from an array.
• groupBy: Collect related values into keyed groups.