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JSON encoding/decoding module for Lua
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JSON encoding/decoding module for Lua

lua-json provides fast JSON encoding/decoding routines for Lua:

  • Support for inline data transformation/filtering via metamethods/handlers.
  • Written in C with 32/64-bit awareness.
  • No external dependencies.

json.encode(value, [event])

Returns a text string containing a JSON representation of value. Optional event may be used to specify a metamethod name (default is __toJSON) that is called for every processed value. The value returned by the metamethod is used instead of the original value.

A table (root or nested) is encoded into a dense array if it has a field __array whose value is true. The length of the resulting array can be adjusted by storing an integer value in that field. Otherwise, it is assumed to be equal to the raw length of the table.

json.decode(data, [pos], [handler])

Returns the value encoded in data along with the index of the first unread byte. Optional pos marks where to start reading in data (default is 1). Optional handler is called for each new table (root or nested), and its return value is used instead of the original table.

When an array is decoded, its length is stored in a field __array.


A Lua value that represents JSON null.

Building and installing with LuaRocks

To build and install, run:

luarocks make

To install the latest release using, run:

luarocks install lua-json

Building and installing with CMake

To build and install, run:

cmake .
make install

To build for a specific Lua version, set USE_LUA_VERSION. For example:

cmake -D USE_LUA_VERSION=5.1 .

or for LuaJIT:

cmake -D USE_LUA_VERSION=jit .

To build in a separate directory, replace . with a path to the source.

Getting started

local json = require 'json'

-- Helpers
local function encode_decode(val, ev, h)
    return json.decode(json.encode(val, ev), nil, h)

-- Primitive types
assert(encode_decode(nil) == json.null)
assert(encode_decode(json.null) == json.null)
assert(encode_decode(false) == false)
assert(encode_decode(true) == true)
assert(encode_decode(123) == 123)
assert(encode_decode(123.456) == 123.456)
assert(encode_decode('abc') == 'abc')

-- Complex types
local data = {
    obj = { -- A table with only string keys translates into an object
        str = 'abc',
        len = 3,
        val = -10.2,
        null = json.null,
    arr1 = {__array = true, 1, 2, 3}, -- A table with a field '__array' translates into an array
    arr2 = {__array = 5, nil, 2, nil, 4, nil}, -- Array length can be adjusted to form a sparse array

local out = encode_decode(data)
assert(out.obj.null == json.null) -- 'null' as a field value
assert(out.arr1.__array == #out.arr1) -- Array length is restored
assert(out.arr2.__array == 5) -- Access to the number of items in a sparse array

-- Serialization metamethods can be used to produce multiple JSON representations of the same object.
-- Deserialization handlers can be used to restore Lua objects from complex JSON types on the way back.
-- This is helpful, for example, when objects are exchanged with both trusted and untrusted parties.
-- Various custom filters/wrappers can also be implemented using this API.

local mt = {
    __tostring = function (t) return (t.a or '') .. (t.b or '') end,
    __toA = function (t) return {A = t.a} end, -- [a -> A]
    __toB = function (t) return {B = t.b} end, -- [b -> B]

local function new(t) return setmetatable(t, mt) end
local function fromA(t) return new{a = t.A} end -- [A -> a]
local function fromB(t) return new{b = t.B} end -- [B -> b]

local obj = new{a = 'a', b = 'b'}
assert(tostring(obj) == 'ab')
assert(tostring(encode_decode(obj, '__toA', fromA)) == 'a')
assert(tostring(encode_decode(obj, '__toB', fromB)) == 'b')

Non-standard JSON values

lua-json recognizes numbers prefixed with 0x as hexadecimal. It also supports the following values: [-]nan, [-]NaN, [-]inf, [-]Infinity.

If strictly compliant JSON generation is preferred, the following technique may be used to filter out these values:

local json = require 'json'

local function filter(t)
    for k, v in pairs(t) do
        if v ~= v or v == 1/0 or v == -1/0 then
            error(("non-standard value '%f' at index '%s'"):format(v, k))
    return t

local mt = {
    __toJSON = function (t)
        return setmetatable(filter(t), nil)

local function check(t)
    return setmetatable(t, mt)

local t = {
    str = 'abc',
    val = 1.234,
    nan = 0/0,
    inf = 1/0,
    ninf = -1/0,

local s = [[{
    "str": "abc",
    "val": 1.234,
    "nan": nan,
    "inf": inf,
    "ninf": -inf

-- Strict encoding

-- Strict decoding
print(json.decode(s, nil, filter))
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