LINQ SelectMany in JavaScript

selectMany.js

Array.prototype.selectMany = function (fn) {
    return this.map(fn).reduce(function (x, y) { return x.concat(y); }, []);
};

// 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; }));

I don't know if you're using this snippet, or if you've rewritten it since or whatever, but I think you can get better performance with concatting all the results at once, rather than using reduce (which creates an array per result).

I created a fork here: https://gist.github.com/sharkbrainguy/5086585
And a jsperf case here: http://jsperf.com/concatenating-many-arrays

It's faster for the use-case where you're returning many results per item for many items (in latest chrome, latest ff, and IE9).

I hope that's helpful :)