What's a good mathematical sets implementation in JavaScript?

intersection, javascript, math, set

Solution

I don't know of any existing implementations, but if your set elements are strings (or have a unique string representation) you can use JavaScript objects pretty easily. The elements would be the object properties, and the value could be anything.

// Make a set from an array of elements
function makeSet(items) {
    var set = {};
    for (var i = 0; i < items.length; i++) {
        set[items[i]] = true;
    }
    return set;
}

function copyInto(s, copy) {
    for (var item in s) {
        if (s[item] === true) {
            copy[item] = true;
        }
    }
}

function union(s1, s2) {
    var u = {};
    copyInto(s1, u);
    copyInto(s2, u);
    return u;
}

function intersection(s1, s2) {
    var i = {};
    for (var item in s1) {
        if (s1[item] === true && s2[item] === true) {
            i[item] = true;
        }
    }
    return i;
}

function difference(s1, s2) {
    var diff = {};
    copyInto(s1, diff);
    for (var item in s2) {
        if (s2[item] === true) {
            delete diff[item];
        }
    }
    return diff;
}

// etc.

You could also use `item in set` or `set.hasOwnProperty(item)` instead of `set[item] === true`, but checking by for `true` explicitly, you automatically ignore any functions that might be attached to the object (in case someone modified Object.prototype, or it's not a plain object).

Problem

Original source