no-unsafe-declaration-merging

Disallow unsafe declaration merging.

🔒

Extending "plugin:@typescript-eslint/strict" in an ESLint configuration enables this rule.

TypeScript's "declaration merging" supports merging separate declarations with the same name.

Declaration merging between classes and interfaces is unsafe. The TypeScript compiler doesn't check whether properties are initialized, which can cause lead to TypeScript not detecting code that will cause runtime errors.

interface Foo {
  nums: number[];
}

class Foo {}

const foo = new Foo();

foo.nums.push(1); // Runtime Error: Cannot read properties of undefined.

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/no-unsafe-declaration-merging": "error"
  }
};

Try this rule in the playground ↗

Examples

❌ Incorrect

interface Foo {}

class Foo {}

✅ Correct

interface Foo {}
class Bar implements Foo {}

namespace Baz {}
namespace Baz {}
enum Baz {}

namespace Qux {}
function Qux() {}

Further Reading

• Declaration Merging

This rule is not configurable.

Options

Resources

• Rule source
• Test source