The __newindex meta method is complementary to the __index meta method. Where __index is used to retrieve values from missing keys in a table, __newindex is used to assign values to missing keys. The __newindex method takes three arguments:

• The table that is being operated on
• The missing key
• The value being assigned

Here is an example of using the __newindex meta method:

x = { }
y = { }

z = {
  __index = function(table, key)
    return z[key]
  end,
  __newindex = function(table, key, value) 
    z[key] = value
  end
}

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

x.foo = "bar"

print (x.foo) 
print (y.foo)
print (z.foo)

In this example, both tables x and y have meta table z. The z meta table has an __index meta method, which searches for a key within z whenever a nonexistent key is accessed. It also has a __newindex meta method, which sets a new key value pair in the z meta table if x or y don't have the key present already.

When executing the line x.foo = "bar", Lua sees that x does not have a foo member, but it does have a meta table with the __newindex meta method present. Lua then calls the __newindex meta method of the meta table (z), which assigns the key value pair to table z.

When printing the foo key of either table x or y, the __index meta method of their meta table, z, is invoked, returning the foo value stored in z.