DefaultingMap

A tiny dependency-free extension to the native Map, with defaults and updates perfect for reducing data into buckets.

Installation

npm install defaulting-map

Why DefaultingMap?

Turn this:

if (!map.has(key)) {
    map.set(key, 0);
}
map.set(key, map.get(key) + 1);

Into this:

map.update(key, (value) => value + 1);

Example

const text = "foo bar foo baz";
const wordCounts = new DefaultingMap(() => 0);

for (const word of words.split(' ')) {
  wordCounts.update(word, (count) => count + 1);
}

console.log([...wordCounts.entries()]); // [ [ 'foo', 2 ], [ 'bar', 1 ], [ 'baz', 1 ] ]

Getting unset values

By default, .get(key) returns undefined for unset keys (like Map). To get the default value instead:

const map = new DefaultingMap(() => 0, { getUnsetAsDefault: true });

console.log(map.get('nonexistent')); // 0

Even with this setting, map.keys(), map.entries() etc will continue to behave like Map and return data for explicitly set keys.