This little module provides an ArrayIndex constructor function that you can inherit from with your own objects. When a numbered property gets read, then the ArrayIndex.get function on the object will be invoked. When a numbered property gets set, then the ArrayIndex.set function on the object will be invoked.

Install with npm:

$ npm install array-index

A quick silly example, using Math.sqrt() for the "getter":

var ArrayIndex = require('array-index');

// let's just create a singleton instance.
var a = new ArrayIndex();

// the "ArrayIndex.get" function is invoked for each "a[n]" access.
// it is given a single argument, the "index" currently being accessed.
// so here, we're passing in the `Math.sqrt()` function, so accessing
// "a[9]" will return `Math.sqrt(9)`.
a[ArrayIndex.get] = Math.sqrt;

// the "ArrayIndex.get" and "ArrayIndex.set" functions are only invoked up
// to "a.length", so we must set that manually.
a.length = 10;

console.log(a);
// [ 0,
//   1,
//   1.4142135623730951,
//   1.7320508075688772,
//   2,
//   2.23606797749979,
//   2.449489742783178,
//   2.6457513110645907,
//   2.8284271247461903,
//   3 ]

Here's an example of creating a subclass of ArrayIndex using util.inherits():

var ArrayIndex = require('array-index');
var inherits = require('util').inherits;

function MyArray (length) {
  // be sure to call the ArrayIndex constructor in your own constructor
  ArrayIndex.call(this, length);

  // the "set" object will contain values at indexes previously set,
  // so that they can be returned in the "getter" function. This is just a
  // silly example, your subclass will have more meaningful logic.
  Object.defineProperty(this, 'set', {
    value: Object.create(null),
    enumerable: false
  });
}

// inherit from the ArrayIndex's prototype
inherits(MyArray, ArrayIndex);

MyArray.prototype[ArrayIndex.get] = function (index) {
  if (index in this.set) return this.set[index];
  return index * 2;
};

MyArray.prototype[ArrayIndex.set] = function (index, v) {
  this.set[index] = v;
};


// and now you can create some instances
var a = new MyArray(15);
a[9] = a[10] = a[14] = '_';
a[0] = 'nate';

console.log(a);
// [ 'nate', 2, 4, 6, 8, 10, 12, 14, 16, '_', '_', 22, 24, 26, '_' ]

The ArrayIndex base class is meant to be subclassed, but it also has a few convenient functions built-in.

The length of the ArrayIndex instance. The ArrayIndex.get and ArrayIndex.set functions will only be invoked on the object up to this "length". You may set this length at any time to adjust the amount range where the getters/setters will be invoked.

Returns a new regular Array instance with the same values that this ArrayIndex class would have. This function calls the ArrayIndex.get function repeatedly from 0...length-1 and returns the "flattened" array instance.

All ArrayIndex instances get basic support for JSON.stringify(), which is the same as a "flattened" Array being stringified.

The toString() override is basically just array.toArray().toString().

The inspect() implementation for the REPL attempts to mimic what a regular Array looks like in the REPL.

(The MIT License)

Copyright (c) 2012 Nathan Rajlich <[email protected]>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.