iterable.js

var Iterable = {
    func: {
        predicate: {
            TRUE: function (x) { return true; }
        }
    }
};

Array.prototype.forEach = function (fn) {
    for (var i = 0; i < this.length; i++)
        fn(this[i]);
};

Array.prototype.filter = function (pred) {
    var result = [];
    this.forEach(function (x) {
        if (pred(x))
            result.push(x);
    });

    return result;
};

Array.prototype.distinct = function () {
    var result = [];
    this.forEach(function (x) {
        if (!result.any(function (y) { return y == x; }))
            result.push(x);
    });

    return result;
};

Array.prototype.any = function (pred) {
    pred = pred || Iterable.func.predicate.TRUE;
    
    for (var i = 0; i < this.length; i++)
        if (pred(this[i]))
            return true;
        
    return false;
};

Array.prototype.indexOf = function (expr) {
    if (!(expr instanceof Function))
        expr = function (x) { return x == expr; };

    for (var i = 0; i < this.length; i++)
        if (expr(this[i]))
            return i;

    return -1;
};

Array.prototype.singleOrNull = function (pred) {
    var filtered = this.filter(pred);
    if (filtered.length == 0)
        return null;
    if (filtered.length > 1)
        throw "More than one element returned.";
    
    return filtered[0];
};

Array.prototype.map = function (fn) {
    var mapped = [];
    this.forEach(function (x) { mapped.push(fn(x)); });
    return mapped;
};

Array.prototype.groupBy = function (sel) {
    var groups = [];

    this.forEach(function (x) {
        var group = groups.singleOrNull(function (y) { return y.key == sel(x); });
        if (!group)
            groups.push({ key: sel(x), members: [x] });
        else
            group.members.push(x);
    });

    return groups;
};