Nginx - multiple/nested IF statements

What I want to do:

• Check if request comes from Facebook
• Check if URL is like domain.com/2
• If above conditions are true - show content from /api/content/item/$1?social=1
• If above conditions are false - show "normal page"

It is a single page app. Before my changes configuration looked like this (and it worked):

location / {
    root /home/eshlox/projects/XXX/project/project/assets/dist;
    try_files $uri $uri/ /index.html =404;
}

I've tried to use if statements:

location / {
    set $social 1;
    if ($http_user_agent ~* "facebookexternalhit") {
        set $social UA; 
    }       
    if ($uri ~* "^/(\d+)$") {
        set $social "${social}URL";
    }       
    if ($social = UAURL) {
        rewrite ^/(\d+)$ /api/content/item/$1?social=1;
    }

    root /home/eshlox/projects/XXX/project/project/assets/dist;
    try_files $uri $uri/ /index.html =404;
}

With this configuration everything works as I expected only if both conditions are true or false. If one of conditions is true and the second is false (or vice versa) then nginx always returns status 404.

I have found "IfIsEvil" on nginx site, I've tried to use mapping (can I use mapping in this case?), but still I can't resolve this problem.

Any ideas?

There is good article about common pitfalls in nignx wiki.

First, I've moved root directive to server level. Second, location is the best way to check urls. So I rethink your requirements as

• if location consist of digits
• and request from facebook

we have to rewrite url, and the result is:

root /home/eshlox/projects/XXX/project/project/assets/dist;

location / {
    try_files $uri $uri/ /index.html;
}

location ~ "^/\d+$" {
    if ($http_user_agent ~* "facebookexternalhit") {
        rewrite (.+) /api/content/item$1?social=1;
    }

    try_files $uri $uri/ /index.html;
}

Also, there is almost no reason to have =404 after /index.html in try_files directive.