selectMany.js

(function () {
    var apply = Function.prototype.apply;
    var flatten = apply.bind(Array.prototype.concat, []);

    Array.prototype.selectMany = function (fn) {
        return flatten(this.map(fn));
    };
}());

// usage

console.log([[1,2,3], [4,5,6]].selectMany(function (x) { return x; })); //[1,2,3,4,5,6]

console.log([{ a: [1,2,3] }, { a: [4,5,6] }].selectMany(function (x) { return x.a; }));

console.log([{ title: 'Some Blog', tags: ['being awesome'] }, { title: 'Some Other Blog', tags: ['still awesome', 'javascript'] } ].selectMany(function (blog) { return blog.tags; }));