index.js

// contains, add, remove, toggle
var indexof = require('indexof')

module.exports = ClassList

function ClassList(elem) {
    var cl = elem.classList

    if (cl) {
        return cl
    }

    var classList = {
        add: add
        , remove: remove
        , contains: contains
        , toggle: toggle
        , toString: $toString
        , length: 0
        , item: item
    }

    return classList

    function add(token) {
        var list = getTokens()
        if (indexof(list, token) > -1) {
            return
        }
        list.push(token)
        setTokens(list)
    }

    function remove(token) {
        var list = getTokens()
            , index = indexof(list, token)

        if (index === -1) {
            return
        }

        list.splice(index, 1)
        setTokens(list)
    }

    function contains(token) {
        return indexof(getTokens(), token) > -1
    }

    function toggle(token) {
        if (contains(token)) {
            remove(token)
            return false
        } else {
            add(token)
            return true
        }
    }

    function $toString() {
        return elem.className
    }

    function item(index) {
        var tokens = getTokens()
        return tokens[index] || null
    }

    function getTokens() {
        var className = elem.className

        return filter(className.split(" "), isTruthy)
    }

    function setTokens(list) {
        var length = list.length

        elem.className = list.join(" ")
        classList.length = length

        for (var i = 0; i < list.length; i++) {
            classList[i] = list[i]
        }

        delete list[length]
    }
}

function filter (arr, fn) {
    var ret = []
    for (var i = 0; i < arr.length; i++) {
        if (fn(arr[i])) ret.push(arr[i])
    }
    return ret
}

function isTruthy(value) {
    return !!value
}