if a then
    ngx.say("hello")
end

You can simplify the operation by changing the tab to 4 spaces in the editor you are using.

Space

On both sides of the operator, you need to use a space to separate:

--No
local i=1
local s    =    "apisix"

--Yes
local i = 1
local s = "apisix"

Blank line

Many developers will add a semicolon at the end of the line:

--No
if a then
    ngx.say("hello");
end;

Adding a semicolon will make the Lua code look ugly and unnecessary. Also, don’t want to save the number of lines in the code, the latter turns the multi-line code into one line in order to appear “simple”. This will not know when the positioning error is in the end of the code:

--No
if a then ngx.say("hello") end

--Yes
if a then
    ngx.say("hello")
end

The functions needs to be separated by two blank lines:

--No
local function foo()
end
local function bar()
end

--Yes
local function foo()
end
local function bar()
end

If there are multiple if elseif branches, they need a blank line to separate them:

--No
if a == 1 then
    foo()
elseif a== 2 then
    bar()
elseif a == 3 then
    run()
else
    error()
end

--Yes
if a == 1 then
    foo()
elseif a == 2 then
    bar()
elseif a == 3 then
    run()
else
    error()
end

Each line cannot exceed 100 characters. If it exceeds, you need to wrap and align:

--No
return limit_conn_new("plugin-limit-conn", conf.conn, conf.burst, conf.default_conn_delay)

--Yes
return limit_conn_new("plugin-limit-conn", conf.conn, conf.burst,
                      conf.default_conn_delay)

If it is a string stitching alignment, you need to put .. in the next line:

return limit_conn_new("plugin-limit-conn" ..  "plugin-limit-conn" ..
                      "plugin-limit-conn")

--Yes
return "param1", "plugin-limit-conn"
                 .. "plugin-limit-conn"

Variable

Local variables should always be used, not global variables:

--No
i = 1
s = "apisix"

local i = 1
local s = "apisix"

Variable naming uses the snake_case style:

--No
local IndexArr = 1
local str_Name = "apisix"

--Yes
local index_arr = 1
local str_name = "apisix"

Use all capitalization for constants:

--No
local max_int = 65535
local server_name = "apisix"

--Yes
local MAX_INT = 65535
local SERVER_NAME = "apisix"

Table

Use table.new to pre-allocate the table:

--No
local t = {}
for i = 1, 100 do
    t[i] = i
end

--Yes
local new_tab = require "table.new"
local t = new_tab(100, 0)
for i = 1, 100 do
    t[i] = i
end

Don’t use nil in an array:

--No
local t = {1, 2, nil, 3}

If you must use null values, use ngx.null to indicate:

--Yes
local t = {1, 2, ngx.null, 3}

--No
local s = ""
for i = 1, 100000 do
    s = s .. "a"
end

--Yes
local new_tab = require "table.new"
local t = new_tab(100, 0)
for i = 1, 100000 do
    t[i] = "a"
end
local s = table.concat(t, "")

Function

The naming of functions also follows snake_case:

--Yes
local function test_nginx()
end

The function should return as early as possible:

--No
local function check(age, name)
    local ret = true
    if age < 20 then
        ret = false
    end
    if name == "a" then
        ret = false
    end
    -- do something else
    return ret
end

--Yes
local function check(age, name)
    if age < 20 then
    end
    if name == "a" then
    end
    -- do something else
    return true
end

The function should return <boolean>, err. The first return value means successful or not, if not, the second return value specifies the error message. The error message can be ignored in some case.

--No
local function check()
    return "failed"
end

--Yes
local function check()
    return false, "failed"
end

Module

All require libraries must be localized:

--No
local function foo()
    local ok, err = ngx.timer.at(delay, handler)
end

--Yes
local timer_at = ngx.timer.at
local function foo()
    local ok, err = timer_at(delay, handler)
end

For style unification, require and ngx also need to be localized:

--No
local core = require("apisix.core")
local timer_at = ngx.timer.at
local function foo()
    local ok, err = timer_at(delay, handler)
end

--Yes
local ngx = ngx
local require = require
local core = require("apisix.core")
local timer_at = ngx.timer.at
local function foo()
    local ok, err = timer_at(delay, handler)
end

For functions that return with error information, the error information must be judged and processed:

--No
local sock = ngx.socket.tcp()
local ok = sock:connect("www.google.com", 80)
ngx.say("successfully connected to google!")

--Yes
local sock = ngx.socket.tcp()
local ok, err = sock:connect("www.google.com", 80)
if not ok then
    ngx.say("failed to connect to google: ", err)
    return
end
ngx.say("successfully connected to google!")

The function you wrote yourself, the error message is to be returned as a second parameter in the form of a string:

--No
local function foo()
    local ok, err = func()
    if not ok then
        return false
    end
    return true
end

--No
local function foo()
    local ok, err = func()
    if not ok then
        return false, {msg = err}
    end
    return true