JavaScript defaultdict

This is a port of python's defaultdict to JavaScript. It uses proxies to create objects that return values other than undefined for attributes that haven't yet been set.

This package needs native Proxy support to work.

Install

yarn add defaultdict-proxy

Usage

If called with undefined, null, a boolean, number, string or symbol, defaultdict returns a proxy object that returns this value for unset attributes. If called with an array or object, accessing unset attributes returns a shallow copy of the array or object. If defaultdict is given a function, it returns a proxy object that has unset attributes initialized to the result of executing the function.

import defaultdict from 'defaultdict-proxy';

// This defaultdict returns fresh empty arrays for unset attributes
const dd1 = defaultdict([]);

// defaultdicts act like normal objects
dd1.a = { b: 0 };
dd1.a.b === 0; // true

// We can use attributes without setting them first
dd1.b.push(42);
dd1.b; // [42]
dd1.c; // []
dd1.d === dd1.e; // false, every attribute gets its own copy

// A defaultdict that initializes unset attributes with the time of their
// first access
const dd2 = defaultdict(() => new Date());

// The attribute initialization function is given the name of the attribute
// as a parameter. This defaultdict initializes unset attributes to their
// name
const dd3 = defaultdict(attr => attr);
dd3.blah // 'blah'

// To create defaultdicts that initialize unset attributes with functions,
// we have to pass defaultdict a higher order function. In this example,
// unset attribute accesses return the identity function. 
const dd4 = defaultdict(() => x => x);

License

MIT