When you try to access a nonexistent field in a table, the result is nil. However, if the table being accessed has a meta table and that meta table has an __index meta method, the meta method will be called.

There are two tables, x and y. Neither table has a hello key, but both tables try to print this key. In both instances, the print statement will result in nil:

x = {
  foo = "bar"
}
y = { }

print (x.foo) -- bar
print (x.hello) -- nil

print (y.foo) -- nil
print (y.hello) -- nil

To fix this, create a meta table with an __index meta method. _index takes two arguments. The first argument is the table that is being indexed, the second argument is the key.

The following code creates two meta tables, z and w; both have an __index meta method. Table z uses this meta method to return a variable found in itself, while w manually checks the key and returns some inline string. Table x is assigned the z meta method, table y is assigned the w meta method:

x = {
  foo = "bar"
}

y = { }

z = {
  hello = "world z",
  __index = function(table, key) 
    return z[key]
  end
}

w = {
  __index = function(table, key) 
    if key == "hello" then
      return "inline world"
    end
    return nil
  end
}

setmetatable(x, z)
setmetatable(y, w)

print (x.foo) -- bar
print (x.hello) -- world z

print (y.foo) -- nil
print (y.hello) -- inline world

In this code, when printing x.foo, the foo field of the x table is printed. When printing x.hello, Lua sees that table x does not have a hello field, but it does have a meta table and that meta table has an __index function. Lua then passes table x and hello as the arguments to the __index function of the meta table, which returns the hello field of
table z.

When printing y.foo, Lua sees that table y does not have a hello field, but it does have a meta table with an __index meta method. The __index meta method of table w is called, with table y and foo as arguments. This meta method only returns a value if the key provided is hello. Since the key provided is not hello, the __index meta method returns nil.