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Like `new Map` but for older JavaScripts
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test test and document initializing Map at construct time Nov 27, 2015
.travis.yml travis Jan 5, 2016
LICENSE isc Nov 27, 2015
map.js avoid return statement in outermost code block Jan 3, 2016
pseudomap.js avoid return statement in outermost code block Jan 3, 2016


A thing that is a lot like ES6 Map, but without iterators, for use in environments where for..of syntax and Map are not available.

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If you need iterators, or just in general a more faithful polyfill to ES6 Maps, check out es6-map.

If you are in an environment where Map is supported, then that will be returned instead, unless process.env.TEST_PSEUDOMAP is set.

You can use any value as keys, and any value as data. Setting again with the identical key will overwrite the previous value.

Internally, data is stored on an Object.create(null) style object. The key is coerced to a string to generate the key on the internal data-bag object. The original key used is stored along with the data.

In the event of a stringified-key collision, a new key is generated by appending an increasing number to the stringified-key until finding either the intended key or an empty spot.

Note that because object traversal order of plain objects is not guaranteed to be identical to insertion order, the insertion order guarantee of Map.prototype.forEach is not guaranteed in this implementation. However, in all versions of Node.js and V8 where this module works, forEach does traverse data in insertion order.


Most of the Map API, with the following exceptions:

  1. A Map object is not an iterator.
  2. values, keys, and entries methods are not implemented, because they return iterators.
  3. The argument to the constructor can be an Array of [key, value] pairs, or a Map or PseudoMap object. But, since iterators aren't used, passing any plain-old iterator won't initialize the map properly.


Use just like a regular ES6 Map.

var PseudoMap = require('pseudomap')

// optionally provide a pseudomap, or an array of [key,value] pairs
// as the argument to initialize the map with
var myMap = new PseudoMap()

myMap.set(1, 'number 1')
myMap.set('1', 'string 1')
var akey = {}
var bkey = {}
myMap.set(akey, { some: 'data' })
myMap.set(bkey, { some: 'other data' })
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