Collection – a tiny, immutable data structure for JavaScript/TypeScript

collection.js

Collection = (spec) => {
  if (typeof spec !== 'object') throw new TypeError('Collection specification must be an object')
  return Object.freeze(Object.create(Collection.prototype, Object.getOwnPropertyDescriptors(spec)))
}
Collection.prototype = Object.create(null, {
  [Symbol.isConcatSpreadable]: { value: true },
  [Symbol.iterator]:           { value: function* () { for (let key in this) { yield this[key] } } },
  [Symbol.toPrimitive]:        { value: (hint) => hint === 'number' ? NaN : '[object Collection]' },
  [Symbol.toStringTag]:        { value: 'Collection' },
})


// Keyed collection
keyed = Collection({ foo: 42, bar: true })
keyed.foo                // 42
keyed.bar                // true
Object.keys(arrayish)    // [ "foo", "bar" ]
Object.values(arrayish)  // [ 42, true ]
({ foo, bar } = keyed)   // foo = 42, bar = true
[first, second] = keyed  // first = 42, second = true

// Indexed collection
indexed = Collection({ 0: 42, 1: true })
indexed[0]                           // 42
indexed[1]                           // true
Object.keys(indexed)                 // [ "0", "1" ]
Object.values(indexed)               // [ 42, true ]
[first, second] = indexed            // first = 42, second = true
({ 0: first, 1: second } = indexed)  // first = 42, second = true

// Mixed collection (Array-like)
arrayish = Collection([42, true])
arrayish[0]                 // 42
arrayish[1]                 // true
arrayish.length             // 2
Object.keys(arrayish)       // [ "0", "1", "length" ]
Object.values(arrayish)     // [ 42, true, 3 ]
[first, second] = arrayish  // first = 42, second = true
({ length } = arrayish)     // length = 2