nginx if condition cant get a correct value from ngx.var.XXX

Hi, I found that nginx if condition cant get a correct value from ngx.var.XXX. The ngx.var.XXX is processed by a lua script. The vhost config is

###################
location / {
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

    index  index.html index.htm;
    lua_code_cache off;

    set $ab_flag "";
    rewrite_by_lua_file "/usr/local/openresty/nginx/nginx_upstream_change.lua";
    if ($ab_flag = "yes") {
      set $ab_flag "yes";
      echo "yes:" $ab_flag;
    }
    if ($ab_flag = "no") {
      set $ab_flag "no";
      echo "no:" $ab_flag;
    }
    echo ":" $ab_flag;
}

###################

The nginx_upstream_change.lua script will return a 'yes' or 'no' value to bind ngx.var.ab_flag variable. Use curl tool to call this vhost url, the repsonse is ": no". So i dont know why if condition is not useful. Thanks.

Start with the vhost configuration in the issue and the referenced nginx_upstream_change.lua script, then reproduce the response with curl. Trace when ngx.var.ab_flag is assigned and when each nginx if condition evaluates it; done means explaining why the conditions do not match the returned value or identifying the missing reproduction detail.