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prefer-ts-extras-object-assign

Prefer objectAssign from ts-extras over Object.assign(...).

objectAssign(...) preserves Object.assign(...) runtime behavior while returning a more conservative and useful merge type for TypeScript.

Targeted pattern scopeโ€‹

This rule focuses on direct Object.assign(target, ...sources) calls that can be migrated to objectAssign(target, ...sources) with deterministic fixes.

  • Object.assign(target, ...sources) call sites that can use objectAssign(target, ...sources).

Alias indirection, wrapper helpers, and non-canonical call shapes are excluded to keep objectAssign(target, ...sources) migrations safe.

What this rule reportsโ€‹

This rule reports Object.assign(target, ...sources) call sites when objectAssign(target, ...sources) is the intended replacement.

  • Object.assign(target, ...sources) call sites that can use objectAssign(target, ...sources).

Why this rule existsโ€‹

Object.assign returns an intersection type for common merge patterns, which can produce misleading never properties when target and source keys conflict. objectAssign keeps the same runtime behavior but uses stronger merge typing.

  • Overlapping keys keep target and source value types visible.
  • Source-only keys are modeled conservatively.
  • Team code converges on one explicit runtime helper for object merging.

โŒ Incorrectโ€‹

const merged = Object.assign({ a: 1 }, { a: "x" });

โœ… Correctโ€‹

const merged = objectAssign({ a: 1 }, { a: "x" });

Behavior and migration notesโ€‹

  • Runtime semantics stay aligned with Object.assign.
  • The target object is still mutated and returned.
  • Source property enumeration behavior remains the same as native Object.assign.
  • Use objectUpdate only when you intentionally want to restrict updates to existing writable target properties; it is not a general replacement for Object.assign.

Additional examplesโ€‹

Incorrect - Additional exampleโ€‹

const settings = Object.assign({}, defaults, overrides);

Correct - Additional exampleโ€‹

const settings = objectAssign({}, defaults, overrides);

Correct - Repository-wide usageโ€‹

const normalized = objectAssign({ enabled: true }, options);

ESLint flat config exampleโ€‹

import typefest from "eslint-plugin-typefest";

export default [
 {
  plugins: { typefest },
  rules: {
   "typefest/prefer-ts-extras-object-assign": "error",
  },
 },
];

When not to use itโ€‹

Disable this rule if you must use native Object.assign directly for interop constraints.

Package documentationโ€‹

ts-extras package documentation:

Source file: source/object-assign.ts

/**
 * A strongly-typed version of `Object.assign()`.
 *
 * This is useful since `Object.assign()` returns the intersection `Target &
 * Source`, which is unsound: conflicting property types are intersected (for
 * example, `{a: number} & {a: string}` makes `a` become `never`), and unsafe
 * access through index signatures is not caught.
 *
 * This function returns a conservative merged type instead, where source-only
 * keys are optional and overlapping keys include the target and source value
 * types.
 *
 * @category Improved builtin
 */

Rule catalog ID: R125

Further readingโ€‹

Adoption resourcesโ€‹