Function each

tools/generate-filesize.js:10–19

const FIXTURES_DIR = `${__dirname}/../test/fixtures`;

const each = fn => arr => {
	arr = Array.isArray(arr) ? arr : [arr];

	return arr
		.reduce(
			(prev, curr, i) => prev.then(() => fn(curr, i, arr.length)),
			Promise.resolve(),
		)
		.then(() => arr);
};

const dirs = fs
	.readdirSync(FIXTURES_DIR)