For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt, and this page is available as Markdown at /reference/object/flattenObject.md.
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    flattenObject es-toolkit

    Converts a nested object into a flat object.

    const flattened = flattenObject(object, options?);

    Usage

    flattenObject(object, options?)

    Use flattenObject when you want to flatten deeply nested objects or arrays using dot (.) notation for keys. Each nested property becomes a single-level object with keys connected by a delimiter.

    import { flattenObject } from 'es-toolkit/object';

// Flatten a nested object
const nestedObject = {
  a: {
    b: {
      c: 1,
    },
  },
  d: [2, 3],
  e: 'simple',
};

const flattened = flattenObject(nestedObject);
console.log(flattened);
// {
//   'a.b.c': 1,
//   'd.0': 2,
//   'd.1': 3,
//   'e': 'simple'
// }

// Use a custom delimiter
const withCustomDelimiter = flattenObject(nestedObject, { delimiter: '/' });
console.log(withCustomDelimiter);
// {
//   'a/b/c': 1,
//   'd/0': 2,
//   'd/1': 3,
//   'e': 'simple'
// }

    It's useful for flattening configuration objects like this:

    // Flatten a configuration object
const config = {
  database: {
    host: 'localhost',
    port: 5432,
    credentials: {
      username: 'admin',
      password: 'secret',
    },
  },
  features: ['auth', 'logging'],
  debug: true,
};

const flatConfig = flattenObject(config);
console.log(flatConfig);
// {
//   'database.host': 'localhost',
//   'database.port': 5432,
//   'database.credentials.username': 'admin',
//   'database.credentials.password': 'secret',
//   'features.0': 'auth',
//   'features.1': 'logging',
//   'debug': true
// }

    Using options.delimiter, you can flatten the object with custom characters like underscores (_) instead of dots (.).

    // Environment variable style with underscores
const envStyle = flattenObject(config, { delimiter: '_' });
console.log(envStyle);
// {
//   'database_host': 'localhost',
//   'database_port': 5432,
//   'database_credentials_username': 'admin',
//   'database_credentials_password': 'secret',
//   'features_0': 'auth',
//   'features_1': 'logging',
//   'debug': true
// }

    Using the preserveArrays option, you can keep arrays as values instead of flattening them into index keys.

    // Keep arrays as values
const preserved = flattenObject(config, { preserveArrays: true });
console.log(preserved);
// {
//   'database.host': 'localhost',
//   'database.port': 5432,
//   'database.credentials.username': 'admin',
//   'database.credentials.password': 'secret',
//   'features': ['auth', 'logging'],
//   'debug': true
// }

    Empty objects and special cases are handled appropriately.

    // Empty objects or arrays
const emptyCase = {
  empty: {},
  emptyArray: [],
  nullValue: null,
  undefinedValue: undefined,
};

const result = flattenObject(emptyCase);
console.log(result);
// {
//   'empty': {},
//   'emptyArray': [],
//   'nullValue': null,
//   'undefinedValue': undefined
// }
// Empty objects and empty arrays appear as keys

    Parameters

    objectobjectrequired

    The object to flatten.

    optionsFlattenObjectOptionsoptional

    Flattening options.

    • delimiter (string, optional): The delimiter to connect nested keys. Defaults to '.'.
    • preserveArrays (boolean, optional): If true, arrays are kept as values instead of being flattened. Defaults to false.

    Returns

    A new object with all nested properties flattened.

    Record<string, any>
    Source: es-toolkit

    Re-exported verbatim from es-toolkit. Implementation, edge cases, and performance behavior are owned upstream. This page mirrors the documentation at the pinned version; the linked source is authoritative.