Stair-Chair[1.12.2] v0.1

var chairs = [
    "minecraft:oak_stairs",
    "minecraft:stone_stairs",
    "minecraft:brick_stairs",
    "minecraft:stone_brick_stairs",
    "minecraft:nether_brick_stairs",
    "minecraft:sandstone_stairs",
    "minecraft:spruce_stairs",
    "minecraft:birch_stairs",
    "minecraft:jjungle_stairs",
    "minecraft:quartz_stairs",
    "minecraft:acacia_stairs",
    "minecraft:dark_oak_stairs",
    "minecraft:red_sandstone_stairs",
    "minecraft:purpur_stairs"
];

var chairs_msg = {
    blocked: "You can't sit on it when it was getting blocked.",
    sitting: "Maybe someone is sitting on it before than you.",
    higher: "You can't sit on it because your leg is too short."
}

function interact(e){
    if(e.type == 2 &&
       chairs.indexOf(e.target.name) != -1 &&
       e.target.metadata < 4 &&
       e.player.mainhandItem.name == "minecraft:air" &&
       e.player.offhandItem.name == "minecraft:air" &&
       ! e.player.isSneaking()
      ){
        e.setCanceled(true);
        if(!e.target.world.getBlock(e.target.x, e.target.y+1, e.target.z).isAir()) {
            e.API.executeCommand(e.target.world, "/title " + e.player.name + " actionbar {\"text\":\"" + chairs_msg.blocked + "\"}");
            return;
        }
        if(e.player.world.getNearbyEntities(e.target.pos, 0, -1).length != 0) {
            e.API.executeCommand(e.target.world, "/title " + e.player.name + " actionbar {\"text\":\"" + chairs_msg.sitting + "\"}");
            return;
        }
        if(e.player.y < e.target.y-1) {
            e.API.executeCommand(e.target.world, "/title " + e.player.name + " actionbar {\"text\":\"" + chairs_msg.higher + "\"}");
            return;
        }
        var chair = e.player.world.createEntity("armor_stand");
        chair.setPosition(e.target.x+0.5, e.target.y+0.35, e.target.z+0.5);
        chair.spawn();
        var nbt = chair.getEntityNbt();
        nbt.setByte("Small", 1);
        nbt.setByte("NoBasePlate", 1);
        nbt.setByte("Invulnerable", 1);
        nbt.setByte("FallFlying", 1);
        nbt.setByte("Marker", 1);
        nbt.setByte("NoGravity", 1);
        nbt.setByte("Invisible", 1);
        chair.setEntityNbt(nbt);
        chair.addRider(e.player);
        if(e.player.tempdata.has("chair")) e.player.tempdata.get("chair").despawn();
        e.player.tempdata.put("chair", chair);
    }
}

function tick(e){
    if(e.player.tempdata.has("chair")){
        var temp = e.player.tempdata.get("chair");
        if(temp.getRiders().length == 0 ||
           chairs.indexOf(e.player.world.getBlock(temp.blockX, temp.blockY, temp.blockZ).name)==-1
          ){
            temp.despawn();
            e.player.tempdata.remove("chair");
        }
    }
}

function logout(e){
    if(e.player.tempdata.has("chair")){
        e.player.tempdata.get("chair").despawn();
        e.player.tempdata.remove("chair");
    }
}