json.encode()

Type Function
Library json.*
Return value String
Revision 2017.3122
Keywords json
See also json.decode()

Overview

Returns the Lua object (table) as a JSON-encoded string. Since items with nil values in a Lua table effectively don't exist, you should use json.null as a placeholder value if you need to preserve array indices in your JSON (see discussion of nullval in json.decode()).

Syntax

json.encode( t [, options] )
t (required)

Table. Lua table.

options (optional)

Table. Lua table containing optional directives to the JSON library:

  • indent — set to true to produce formatted JSON output.
  • exception — function called to handle unsupported datatypes. Parameters are reason, value, state and defaultmessage. Return a string to be included in the output or nil to raise an error. By default, Corona handles unsupported values as strings.

Conversion

  1. Encodable Lua types include String, Number, Boolean, Table, or nil.
  2. Unencodable types are noted as a descriptive string in the output.
  3. All control chars are encoded to \uXXXX format, for example "\021" encodes to "\u0015".
  4. All JSON \uXXXX chars are decoded to chars (0-255 byte range only).
  5. JSON single line // and /* */ block comments are discarded during decoding.
  6. Numerically indexed Lua arrays are encoded to JSON lists, for example [1,2,3].
  7. Lua dictionary tables are converted to JSON objects, for example {"one":1,"two":2}.
  8. By default, JSON nulls are decoded to Lua nil and treated by Lua in the normal way (for example, they appear not to exist — see json.decode()).

Examples

General
local json = require( "json" )

local t = {
    ["name1"] = "value1",
    ["name2"] = { 1, false, true, 23.54, "a \021 string" },
    name3 = json.null
}

local encoded = json.encode( t )
print( encoded )  --> {"name1":"value1","name3":null,"name2":[1,false,true,23.54,"a \u0015 string"]}

local encoded = json.encode( t, { indent=true } )
print( encoded )
--> {
-->   "name1":"value1",
-->   "name3":null,
-->   "name2":[1,false,true,23.54,"a \u0015 string"]
--> }

-- Since this was just encoded using the same library it's unlikely to fail
-- However, it's good practice to handle errors anyway
local decoded, pos, msg = json.decode( encoded )
if not decoded then
    print( "Decode failed at " .. tostring(pos) .. ": " .. tostring(msg) )
else
    print( decoded.name2[4] )  --> 23.54
end
Console Debugging
local json = require( "json" )

local function onTapLogo( event )
    print( "onTapLogo: event: " .. json.encode( event, { indent=true } ) )
end

local logo = display.newImageRect( "logo.png", 128, 128 )
logo.x = display.contentCenterX
logo.y = display.contentCenterY
logo:addEventListener( "tap", onTapLogo )
Debug Result
onTapLogo: event: {
    "y":175,
    "x":198,
    "time":2720.303,
    "target":{
        "_dispatchingEventName":"tap",
        "_class":{
            "removeEventListener":"<type 'function' is not supported by JSON.>",
            "addEventListener":"<type 'function' is not supported by JSON.>",
            "__index":"<reference cycle>"
        },
        "_proxy":"<type 'userdata' is not supported by JSON.>",
        "_functionListeners":{
            "tap":["<type 'function' is not supported by JSON.>"]
        }
    },
    "numTaps":1,
    "name":"tap"
}