A generalized map in ES5 based on linear lookup on strict comparison

createMap.js

'use strict';

var assert = require('assert');

module.exports = function(createDefaultValue) {
  var map = {};

  var keyValuePairs = [];

  var lookup = function(key) {
    var matches = keyValuePairs.filter(function(kv) {
      return kv.key === key;
    });

    assert(matches.length <= 1);

    return matches;
  };

  map.get = function(key) {
    var matches = lookup(key);

    if (matches.length === 0) {
      var kv = {
        key: key,
        value: createDefaultValue();
      };

      keyValuePairs.push(kv);

      return kv.value;
    };

    return matches[0].value;
  };

  map.set = function(key, value) {
    var matches = lookup(key);

    if (matches.length === 0) {
      keyValuePairs.push({
        key: key,
        value: value
      });
    } else {
      matches[0].value = value;
    }
  };
  
  return map;
};