a JavaScript hash-map kind of thing

annotated.js

M = function(
    a // placeholder
) {
    a = {}; // initialize buckets. each bucket is an array with even indexes for keys, odd indexes for values

    // API is a single function for both get(key) and set(key, value)
    return function(
        k, // key (any JavaScript value)
        v, // value (optional, triggers set)
        b, // placeholder
        i, // placeholder
        u, // shortcut for undefined
    ) {
        // get existing or new bucket by coercing key to a string and looking up in the buckets object
        b = a[k] = a[k] || [];

        // look for the matching key in the bucket
        // if not found the index will be the next available slot
        for (i = 0; i < b.length && b[i] !== k; i += 2);

        // if value is not undefined
        if (v !== u) {
            b[i] = k;       // set the key
            b[i + 1] = v    // set the value
        }

        // return the value
        return b[i + 1]
    }
}

// Issues:
// * undefined value not possible
// * keys named "hasOwnProperty", etc

index.js

M=function(a){a={};return function(k,v,b,i,u){b=a[k]=a[k]||[];for(i=0;i<b.length&&b[i]!==k;i+=2);if(v!==u){b[i]=k;b[i+1]=v}return b[i+1]}}

LICENSE.txt

Copyright (c) 2011 Tom Robinson, http://tlrobinson.net/
 
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.

package.json

{
  "name": "itty-bitty-hash-map",
  "keywords": [ "hashmap", "map", "hash" ]
}

test.js

require("./index");

var m = M();

var testCases = [
    [0,0],
    [{},1],
    [{},2],
    ["foo",3],
    ["bar",4],
    [null,5],
    [false,6],
    [true,7],
    [function x(){},8]
]

testCases.forEach(function(t) {
    m(t[0], t[1]);
    assert(m(t[0]) === t[1]);
})
testCases.forEach(function(t) {
    assert(m(t[0]) === t[1]);
})

testCases.forEach(function(t) {
    m(t[1], t[0]);
    assert(m(t[1]) === t[0]);
})
testCases.forEach(function(t) {
    assert(m(t[1]) === t[0]);
})

testCases.forEach(function(t) {
    m(t[0], t[0]);
    assert(m(t[0]) === t[0]);
})
testCases.forEach(function(t) {
    assert(m(t[0]) === t[0]);
})

function assert(condition) {
    if (!condition) throw new Error("Assertion Error");
}

you're right. weird how i didn't notice it. that's how i've used my own implementation of a similar idea, but you're right, with this one it sets the content to [0, 1, 2, 3]... it's nice to be able to map over it.

made my own! https://gist.github.com/996085