@notwoods/default-map

A Map subclass that automatically adds missing values

import { DefaultMap } from "@notwoods/default-map";

const map = new DefaultMap(() => []);
map.get("key").push("array-item");

Includes TypeScript support and default arguments in just 281 bytes!

Inspired by Python's defaultdict.

Install

npm install @notwoods/default-map

Usage

Using arrays with the defaultFactory, it is easy to group a sequence of key-value pairs into a map of arrays:

import { DefaultMap } from "@notwoods/default-map";

const pairs = [
  { key: "yellow", value: 1 },
  { key: "blue", value: 2 },
  { key: "yellow", value: 3 },
  { key: "blue", value: 4 },
  { key: "red", value: 1 },
];
const map = new DefaultMap(() => []);
for (const { key, value } of pairs) {
  map.get(key).push(value);
}

Object.fromEntries(map);
// { blue: [2, 4], red: [1], yellow: [1, 3] }

When each key is encountered for the first time, it is not already in the mapping; so an entry is automatically created using the defaultFactory function which returns an empty array. The array.push() operation then attaches the value to the new array. When keys are encountered again, the look-up proceeds normally (returning the array for that key) and the array.push() operation adds another value to the array. This technique is simpler than an equivalent technique using a Map:

const map = new Map();
for (const { key, value } of pairs) {
  if (map.has(key)) {
    const array = map.get(key);
    array.push(value);
  } else {
    map.set(key, [value]);
  }
}

Setting the defaultFactory to return an int makes the DefaultMap useful for counting (like a bag or multiset in other languages):

import { DefaultMap } from "@notwoods/default-map";

const string = "mississippi";
const map = new DefaultMap(() => 0);
for (const char of string) {
  map.get(char) += 1;
}

Object.fromEntries(map);
// { m: 1, i: 4, p: 2, s: 4 }

When a letter is first encountered, it is missing from the mapping, so the defaultFactory function supplies a default count of zero. The increment operation then builds up the count for each letter.

Setting the defaultFactory to return a Set makes the DefaultMap useful for building a dictionary of sets:

import { DefaultMap } from "@notwoods/default-map";

const pairs = [
  { key: "red", value: 1 },
  { key: "blue", value: 2 },
  { key: "red", value: 3 },
  { key: "blue", value: 4 },
  { key: "red", value: 1 },
  { key: "blue", value: 4 },
];
const map = new DefaultMap(() => new Set());
for (const { key, value } of pairs) {
  map.get(key).add(value);
}

Object.fromEntries(map);
// { blue: Set([2, 4]), red: Set([1, 3]) }

API

Most functions are the same as a regular Map.

new DefaultMap(defaultFactory, entries)

Creates a new DefaultMap. Both arguments are optional.

• defaultFactory: a function that returns the default value for an entry. It takes no arguments.
• entries: an array or iterable of key-value pairs to initialize the map with.

.get(key)

Returns the value corresponding to key, or adds a new default value to the map using the defaultFactory function.

.defaultFactory

This attribute is initialized from the first argument to the constructor, if present, or to undefined, if absent. .get(key) will throw an error if .defaultFactory is undefined.

Maintainers

• Tiger Oakes