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Overview

The ParseJSONResultsPlugin automatically parses JSON strings in query results into JavaScript objects and arrays. This is useful with database dialects that don’t automatically parse JSON but return JSON strings instead.

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Installation

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Global Installation

import { Kysely, ParseJSONResultsPlugin } from 'kysely'

const db = new Kysely<Database>({
  dialect,
  plugins: [new ParseJSONResultsPlugin()]
})

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Per-Query Installation

import { ParseJSONResultsPlugin } from 'kysely'

const result = await db
  .selectFrom('person')
  .selectAll()
  .withPlugin(new ParseJSONResultsPlugin())
  .execute()

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Constructor Options

interface ParseJSONResultsPluginOptions {
  objectStrategy?: 'in-place' | 'create'
}

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objectStrategy

Type: 'in-place' | 'create'
Default: 'in-place' Controls how arrays and objects are handled during parsing:
  • 'in-place' - Arrays’ and objects’ values are parsed in-place. This is the most time and space efficient option, but can result in runtime errors if some objects/arrays are readonly.
  • 'create' - New arrays and objects are created to avoid readonly errors.
Example: 
// Most efficient, but may fail with readonly objects
const plugin1 = new ParseJSONResultsPlugin({ objectStrategy: 'in-place' })

// Creates new objects, safer with readonly data
const plugin2 = new ParseJSONResultsPlugin({ objectStrategy: 'create' })
Source: ~/workspace/source/src/plugin/parse-json-results/parse-json-results-plugin.ts:11

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Usage Example

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With SQLite JSON Functions

import * as Sqlite from 'better-sqlite3'
import { Kysely, ParseJSONResultsPlugin, SqliteDialect } from 'kysely'
import { jsonArrayFrom } from 'kysely/helpers/sqlite'

interface Database {
  person: {
    id: number
    first_name: string
    last_name: string
  }
  pet: {
    id: number
    owner_id: number
    name: string
    species: string
  }
}

const db = new Kysely<Database>({
  dialect: new SqliteDialect({
    database: new Sqlite(':memory:'),
  }),
  plugins: [new ParseJSONResultsPlugin()],
})

const result = await db
  .selectFrom('person')
  .select((eb) => [
    'id',
    'first_name',
    'last_name',
    jsonArrayFrom(
      eb.selectFrom('pet')
        .whereRef('owner_id', '=', 'person.id')
        .select(['name', 'species'])
    ).as('pets')
  ])
  .execute()

// Without the plugin, result.pets would be a JSON string
// With the plugin, result.pets is an array of objects
console.log(result[0].pets) // [{ name: 'Fluffy', species: 'cat' }, ...]

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Per-Query Usage

import { ParseJSONResultsPlugin } from 'kysely'
import { jsonArrayFrom } from 'kysely/helpers/sqlite'

// Only this query will have JSON parsed
const result = await db
  .selectFrom('person')
  .select((eb) => [
    'id',
    'first_name',
    'last_name',
    jsonArrayFrom(
      eb.selectFrom('pet')
        .whereRef('owner_id', '=', 'person.id')
        .select(['name', 'species'])
    ).as('pets')
  ])
  .withPlugin(new ParseJSONResultsPlugin())
  .execute()

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Methods

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transformQuery

transformQuery(args: PluginTransformQueryArgs): RootOperationNode
No-op implementation. This plugin doesn’t modify queries, only results. Returns: The original, unmodified query node Source: ~/workspace/source/src/plugin/parse-json-results/parse-json-results-plugin.ts:79

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transformResult

async transformResult(
  args: PluginTransformResultArgs
): Promise<QueryResult<UnknownRow>>
Recursively parses all JSON strings in the result rows into JavaScript objects and arrays. Parameters:
  • args.queryId - Unique identifier for the query
  • args.result - The query result containing rows to parse
Returns: The result with all JSON strings parsed into objects/arrays Source: ~/workspace/source/src/plugin/parse-json-results/parse-json-results-plugin.ts:83

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How It Works

The plugin recursively processes all values in the result rows:
  1. String Detection: Checks if a value is a string starting with [ or {
  2. JSON Parsing: Attempts to parse the string as JSON
  3. Recursive Processing: Recursively processes nested objects and arrays
  4. Type Preservation: Non-JSON values are left unchanged

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Parsing Logic

// Simplified version of the parsing logic

function parse(obj: unknown): unknown {
  if (typeof obj === 'string') {
    // Only try to parse strings that look like JSON
    if (obj.match(/^[\[\{]/)) {
      try {
        return parse(JSON.parse(obj))
      } catch {
        return obj  // Not valid JSON, return as-is
      }
    }
    return obj
  }
  
  if (Array.isArray(obj)) {
    return obj.map(item => parse(item))
  }
  
  if (isPlainObject(obj)) {
    const result = {}
    for (const key in obj) {
      result[key] = parse(obj[key])
    }
    return result
  }
  
  return obj
}

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Use Cases

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SQLite with JSON Functions

SQLite’s json_array() and json_object() functions return JSON strings: 
const result = await db
  .selectFrom('person')
  .select((eb) => [
    eb.fn('json_object', [
      'firstName', 'first_name',
      'lastName', 'last_name'
    ]).as('personData')
  ])
  .withPlugin(new ParseJSONResultsPlugin())
  .execute()

// personData is a JavaScript object, not a string
console.log(result[0].personData.firstName)

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MySQL JSON Columns

MySQL may return JSON columns as strings in some configurations: 
const db = new Kysely<Database>({
  dialect: new MysqlDialect({ /* ... */ }),
  plugins: [new ParseJSONResultsPlugin()]
})

const result = await db
  .selectFrom('products')
  .select(['id', 'name', 'metadata'])  // metadata is a JSON column
  .execute()

// metadata is automatically parsed
console.log(result[0].metadata.category)

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Nested JSON Aggregations

import { jsonArrayFrom, jsonObjectFrom } from 'kysely/helpers/sqlite'

const result = await db
  .selectFrom('author')
  .select((eb) => [
    'id',
    'name',
    jsonArrayFrom(
      eb.selectFrom('book')
        .whereRef('author_id', '=', 'author.id')
        .select((eb) => [
          'id',
          'title',
          jsonArrayFrom(
            eb.selectFrom('review')
              .whereRef('book_id', '=', 'book.id')
              .select(['rating', 'comment'])
          ).as('reviews')
        ])
    ).as('books')
  ])
  .withPlugin(new ParseJSONResultsPlugin())
  .execute()

// Deeply nested JSON is all parsed
console.log(result[0].books[0].reviews[0].rating)

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Performance Considerations

  • In-place Strategy: Fastest and most memory-efficient, suitable for most use cases
  • Create Strategy: Slightly slower but safer when dealing with readonly objects or when you need to preserve original data
  • Large Results: Parsing large JSON results can be CPU-intensive; consider using the plugin only for queries that actually return JSON

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See Also

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