flat_tree.js

// based off of https://github.com/bor0/misc/blob/master/2017/tree.js
// example: https://github.com/bor0/misc/blob/master/2017/tree_test.js

var tree = {
	v: 1,
	l: {
		v: 2,
		l: {
			v: 3,
			l: { v: 4 },
			r: { v: 5 },
		},
		r: { v: 6 },
	},
	r: {
		v: 7,
		l: {
			v: 8,
			l: { v: 9 },
			r: { v: 10 },
		},
		r: { v: 11 },
	}
};

let flat_tree = function(tree) {
	let S = [tree], l = [];

	while (S.length) {
		let v = S.pop();

		l.push(v.v);

		if (v.r) S.push(v.r);
		if (v.l) S.push(v.l);
	}

	return l;
};

console.log(flat_tree(tree));

More generalized version:

let fold_tree = function(tree, cb) {
	let S = [tree];
	var l;

	while (S.length) {
		let v = S.pop();

		l = cb(v, S, l);

		if (v.r) S.push(v.r);
		if (v.l) S.push(v.l);
	}

	return l;
};

Usage example for callback:

let flat_tree = function(node, stack, accumulator) {
	accumulator = accumulator || [];
	accumulator.push(node.v);
	return accumulator;
};