A strict and fast JSON parser/decoder/encoder written in pure Lua.
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Latest commit ff85220 Sep 7, 2017




Lunajson features SAX-style JSON parser and simple JSON decoder/encoder. It is tested on Lua 5.1, Lua 5.2, Lua 5.3, and LuaJIT 2.0. It is written only in pure Lua and has no dependencies. Even so, decoding speed matches lpeg-based JSON implementations because it is carefully optimized. The parser and decoder reject input that is not conformant to the JSON specification (ECMA-404), and the encoder always yields conformant output. The parser and decoder also handle UTF/Unicode surrogate pairs correctly.


luarocks install lunajson

Or you can download source manually and copy src/* into somewhere on your package.path.

Simple Usage

local lunajson = require 'lunajson'
local jsonstr = '{"Hello":["lunajson",1.5]}'
local t = lunajson.decode(jsonstr)
print(t.Hello[2]) -- prints 1.5
print(lunajson.encode(t)) -- prints {"Hello":["lunajson",1.5]}


lunajson.decode(jsonstr, [pos, [nullv, [arraylen]]])

Decode jsonstr. If pos is specified, it starts decoding from pos until the JSON definition ends, otherwise the entire input is parsed as JSON. null inside jsonstr will be decoded as the optional sentinel value nullv if specified, and discarded otherwise. If arraylen is true, the length of an array ary will be stored in ary[0]. This behavior is useful when empty arrays should not be confused with empty objects.

This function returns the decoded value if jsonstr contains valid JSON, otherwise an error will be raised. If pos is specified it also returns the position immediately after the end of decoded JSON.

lunajson.encode(value, [nullv])

Encode value into a JSON string and return it. If nullv is specified, values equal to nullv will be encoded as null.

This function encodes a table t as a JSON array if a value t[1] is present or a number t[0] is present. If t[0] is present, its value is considered as the length of the array. Then the array may contain nil and those will be encoded as null. Otherwise, this function scans non nil values starting from index 1, up to the first nil it finds. When the table t is not an array, it is an object and all of its keys must be strings.

lunajson.newparser(input, saxtbl)

lunajson.newfileparser(filename, saxtbl)

Create and return a sax-style parser context, which parses input or a file specified by filename. input can be a string to be parsed, or a function that returns the next chunk of a data as a string to be parsed (or nil when all data is yielded). An example function for input follows (this sample is essentially same as the implementation of newfileparser). Note that input will never be called once it has returned nil.

local fp = io.open("myfavorite.json")
local function input()
	local s
	if fp then
		s = fp:read(8192)
		if not s then
			fp = nil
	return s

saxtbl is a table of callbacks. It can have the following functions. Those functions will be called on corresponding events, if it is in the table.

  • startobject()
  • key(s)
  • endobject()
  • startarray()
  • endarray()
  • string(s)
  • number(n)
  • boolean(b)
  • null()

A parser context maintains the current parse position, initially 1.


Start parsing from current position. If valid JSON is parsed, the position moves to just after the end of this JSON. Otherwise it errors.


Return current position.


Return the byte of current position as a number. If input is ended, it returns nil. It does not change current position.


Return the n-length string starting from current position, and increase the index by n. If the input ends, the returned string and the updated position will be truncated.


Following graphs are the results of the benchmark, decoding simple.json (about 750KiB) 100 times and encoding simple.lua (the decoded result of simple.json) 100 times. I conducted benchmarks of lunajson 1.0, dkjson 2.5 and Lua CJSON 2.1.0. Dkjson is a popular JSON encoding/decoding library in Lua, which is written in Lua and optionally uses lpeg to spped up decoding. Lua CJSON is a JSON encoding/decoding library implemented in C and is inherently fast.

The graph of decoding benchmark results

The graph of encoding benchmark results

This benchmark is conducted in my desktop machine that equips Core i5 3550K and DDR3-1600 memory. Lua implementations and concerning modules are compiled by GCC 4.9.2 with -O2 -march=ivybridge -mtune=ivybridge options. The versions of lua implementations are the newest official releases at the time of benchmark. The version of lpeg is 0.12.2.

In this benchmark Lunajson performs well considering that it is implemented only in standard Lua, especially in LuaJIT 2.0 benchmark. Lunajson also supplies incremental parsing in a SAX-style API, therfore you don't have to load whole large JSON files into memory in order to scan the information you're interested in from them. Lunajson is especially useful when non-standard libraries cannot be used easily or incremental parsing is favored.