JSON

Use the JSON module to encode and decode Lua tables to and from JSON strings.

Note: For faster performance, and to avoid raising execution count errors with large amounts of data, use RapidJSON instead.

To use the JSON module, add the following line to your script:

require("json")

json.encode()

Returns the Lua object JSON encoded into a string.

Syntax

json.encode( lua_object )

Example

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require("json")
print (json.encode( { 1, 2, 'fred', {first='mars',second='venus',third='earth'} } ))

Debug Output

[1,2,"fred", {"first":"mars","second":"venus","third","earth"}]

json.decode()

Decodes the JSON encoded data structure, and returns a Lua object with the appropriate data.

Syntax

json.decode( json_string )

Example

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require("json")

tbl = {"Thing 1", "Thing 2"}
jsonstr = json.encode(tbl)
print(jsonstr)
tbl2 = json.decode(jsonstr)

print(table.concat(tbl2,","))

Debug Output

["Thing 1","Thing 2"]
Thing 1,Thing 2

json.null()

Returns a unique value that will be encoded as a null in a JSON encoding.

Example

This is necessary in one situation. In Lua, if a key in a table has a nil value, the key is simply discarded (since any non-existent key has a nil value). The encoding of arrays has been built to manage this nil-values in arrays, but associative arrays provide a problem. For example:

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t = { user="test", password=nil }

Since Lua simply discards the password key, JSON4Lua encodes this as the JSON string.

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{"user":"test"}

If your system requires a defined null value, use the following code:

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t = { user="test", password=json.null }

This will now correctly encode to:

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{"user":"test","password":null}

Incidentally, json.null is simply a function that returns itself, so that you can use either json.null or json.null() as desired.

Portions of this topic are reprinted under permission of the JSON4Lua license.