const tree = {
	value: 6,
	left: {
		value: 4,
		left: {
			value: 1,
			left: null,
			right: null
		},
		right: {
			value: 5,
			left: null,
			right: null
		}
	},
	right: {
		value: 8,
		left: null,
		right: {
			value: 9,
			left: null,
			right: null
		}
	}
}

const check = tree => {
	const current = tree
	const expression = current.value > current.left.value && current.value < current.right.value

	if (!current.left) return true
	if (!expressiion) return false

	return check(current.left)
}

console.log(check(tree))