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deepMerge / shallowMerge ​

Problem ​

You need to combine configuration objects, handling nested keys correctly — without stomping nested objects the way Object.assign does.

Solution ​

Use deepMerge(...items) for recursive merging. Pass { arrayStrategy: 'concat' } as the last argument to concatenate arrays. Use shallowMerge(...items) for a single-level Object.assign-style merge.

ts
import { deepMerge } from '@vielzeug/arsenal';

const base = { api: { host: 'localhost', port: 3000 }, retries: 3 };
const overrides = { api: { port: 4000 }, timeout: 5_000 };

deepMerge(base, overrides);
// { api: { host: 'localhost', port: 4000 }, retries: 3, timeout: 5_000 }

Concatenate arrays instead of replacing them ​

ts
import { deepMerge } from '@vielzeug/arsenal';

const a = { tags: ['ts', 'node'] };
const b = { tags: ['vue'] };

deepMerge(a, b, { arrayStrategy: 'concat' });
// { tags: ['ts', 'node', 'vue'] }

Merge three or more objects ​

ts
import { deepMerge, shallowMerge } from '@vielzeug/arsenal';

deepMerge({ a: { x: 1 } }, { a: { y: 2 } }, { b: 3 });
// { a: { x: 1, y: 2 }, b: 3 }

shallowMerge({ a: 1, b: 2 }, { b: 3, c: 4 }, { d: 5 });
// { a: 1, b: 3, c: 4, d: 5 }

Pitfalls ​

  • deepMerge replaces arrays by default — pass { arrayStrategy: 'concat' } as the last argument to concatenate.
  • Later arguments win on key conflicts for both functions.
  • The options object { arrayStrategy: 'concat' | 'replace' } must be the last positional argument and must contain only that key.