JSON Encoding

Encode Lua tables to JSON and decode JSON strings to Lua values. Includes JSON Schema validation for data verification and API contract enforcement.

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local json = require("json")

Encoding

Encode Value

Encodes a Lua value into a JSON string.

-- Simple values
json.encode("hello")        -- '"hello"'
json.encode(42)             -- '42'
json.encode(true)           -- 'true'
json.encode(nil)            -- 'null'

-- Arrays (sequential numeric keys)
json.encode({1, 2, 3})      -- '[1,2,3]'
json.encode({"a", "b"})     -- '["a","b"]'

-- Objects (string keys)
local user = {name = "Alice", age = 30}
json.encode(user)           -- '{"name":"Alice","age":30}'

-- Nested structures
local order = {
    id = "ord-123",
    items = {
        {sku = "ABC", qty = 2},
        {sku = "XYZ", qty = 1}
    },
    total = 99.50
}
json.encode(order)
-- '{"id":"ord-123","items":[{"sku":"ABC","qty":2},{"sku":"XYZ","qty":1}],"total":99.5}'

Returns: string, error

Encoding rules:

• nil becomes null
• Empty tables become [] (or {} if created with string keys)
• Tables with sequential 1-based keys become arrays
• Tables with string keys become objects
• Mixed numeric and string keys cause an error
• Sparse arrays (gaps in indices) cause an error
• Inf/NaN numbers become null
• Recursive table references cause an error
• Maximum nesting depth is 128 levels

Decoding

Decode String

Decodes a JSON string into a Lua value.

-- Parse object
local user, err = json.decode('{"name":"Bob","active":true}')
if err then
    return nil, err
end
print(user.name)    -- "Bob"
print(user.active)  -- true

-- Parse array
local items = json.decode('[10, 20, 30]')
print(items[1])     -- 10
print(#items)       -- 3

-- Parse nested data
local response = json.decode([[
{
    "status": "ok",
    "data": {
        "users": [
            {"id": 1, "name": "Alice"},
            {"id": 2, "name": "Bob"}
        ]
    }
}
]])
print(response.data.users[1].name)  -- "Alice"

-- Handle errors
local data, err = json.decode("not valid json")
if err then
    print(err:kind())     -- "INTERNAL"
    print(err:message())  -- parse error details
end

Returns: any, error

Schema Validation

Validate Value

Validates a Lua value against a JSON Schema. Use this to enforce API contracts or validate user input.

-- Define a schema
local user_schema = {
    type = "object",
    properties = {
        name = {type = "string", minLength = 1},
        email = {type = "string", format = "email"},
        age = {type = "integer", minimum = 0, maximum = 150}
    },
    required = {"name", "email"}
}

-- Valid data passes
local valid, err = json.validate(user_schema, {
    name = "Alice",
    email = "alice@example.com",
    age = 30
})
print(valid)  -- true

-- Invalid data fails with details
local valid, err = json.validate(user_schema, {
    name = "",
    email = "not-an-email"
})
if not valid then
    print(err:message())  -- validation error details
end

-- Schema can also be a JSON string
local schema_json = '{"type":"number","minimum":0}'
local valid = json.validate(schema_json, 42)

Returns: boolean, error

Schemas are cached by content hash for performance.

Validate JSON String

Validates a JSON string against a schema without decoding first. Useful when you need to validate before parsing.

local schema = {
    type = "object",
    properties = {
        action = {type = "string", enum = {"create", "update", "delete"}}
    },
    required = {"action"}
}

-- Validate raw JSON from request body
local body = '{"action":"create","data":{}}'
local valid, err = json.validate_string(schema, body)
if not valid then
    return nil, errors.new("INVALID", "Invalid request: " .. err:message())
end

-- Now safe to decode
local request = json.decode(body)

Returns: boolean, error

Errors

See Error Handling for working with errors.