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typescript/no-unsafe-object-assign

Disallow Object.assign into readonly-typed targets.

Targeted pattern scope​

⚠️ This rule requires type information to run. Configure type-aware linting (parserOptions.project or projectService) before enabling it.

This rule targets calls matching Object.assign(...) and checks the first argument's resolved TypeScript type.

What this rule reports​

This rule uses TypeScript type information to detect Object.assign calls where the target type has readonly properties.

Why this rule exists​

It prevents mutating targets whose type advertises readonly properties.

❌ Incorrect​

type Target = { readonly x: number };
const target: Target = { x: 1 };
Object.assign(target, { x: 2 });

✅ Correct​

type Target = { x: number };
const target: Target = { x: 1 };
Object.assign(target, { x: 2 });

Behavior and migration notes​

This rule reports only and does not provide an autofix.

Prefer immutable copy patterns or avoid readonly target types when mutation is required.

Options​

This rule has no options.

Additional examples​

const mutable = { count: 0 };
Object.assign(mutable, { count: 1 });
// ✅ valid

const readonlyTarget: Readonly<{ count: number }> = { count: 0 };
Object.assign(readonlyTarget, { count: 1 });
// ❌ reported

ESLint flat config example​

import etcMisc from "eslint-plugin-etc-misc";

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/typescript/no-unsafe-object-assign": "error",
  },
 },
];

When not to use it​

Disable this rule if readonly object mutation via Object.assign is intentionally allowed.

Package documentation​

Rule catalog ID: R101

Further reading​

Adoption resources​

  • Start at warning level in CI, then move to error after cleanup.
  • Use focused codemods/autofix batches per package or directory.