Implementation of JavaScript array.indexOf() in Lua. Also, adds the function to Lua's table library

table_indexOf.lua

-- table.indexOf( array, object ) returns the index
-- of object in array. Returns 'nil' if not in array.
table.indexOf = function( t, object )
	local result

	if "table" == type( t ) then
		for i=1,#t do
			if object == t[i] then
				result = i
				break
			end
		end
	end

	return result
end

-- Example Usage:
local t = {1,3,5,7,9}
print( table.indexOf( t, 9 ) ) -- output: 5

Improving on dragonworx's code

function table.indexOf(t, object)
    if type(t) ~= "table" then error("table expected, got " .. type(t), 2) end

    for i, v in pairs(t) do
        if object == v then
            return i
        end
    end
end

Returns nil if it can't find it, so you don't have to check i == -1. With this you only have to check for i