An issue is raised when using a fallback empty object with the logical OR (||) or nullish coalescing (??) operators when spreading values in object literals.

Why is this an issue?

When spreading values in JavaScript object literals, falsy values like null, undefined, false, 0, or empty strings are safely ignored and don’t add any properties to the resulting object. This means that fallback patterns like {…​(foo || {})} or {…​(foo ?? {})} are unnecessary.

The JavaScript spread operator in object literals has built-in safety for falsy values. When you write {…​null} or {…​undefined}, no properties are added to the object, and no error is thrown. This makes defensive fallback objects redundant.

Using unnecessary fallbacks creates several problems:

• Code bloat: The extra fallback object adds unnecessary characters and complexity
• Reduced readability: The defensive pattern suggests there might be a real risk when there isn’t one
• Maintenance overhead: More code to read, understand, and potentially modify
• Performance impact: Creating an empty object as a fallback has a small but unnecessary runtime cost

The pattern likely comes from developers being overly cautious or from experience with array spreading, where […​null] would indeed throw an error. However, object spreading behaves differently and is more forgiving.

What is the potential impact?

This issue primarily affects code maintainability and readability. While not a security or reliability concern, unnecessary fallback objects make the codebase more verbose and can mislead other developers about the actual behavior of the spread operator in object literals.

How to fix?

Remove the unnecessary fallback empty object when spreading in object literals. The spread operator safely handles falsy values without throwing errors.

Non-compliant code example

const object = {...(foo || {})}; // Noncompliant

Compliant code example

const object = {...foo};

Documentation

• eslint-plugin-unicorn - Rule no-useless-fallback-in-spread
• MDN - Spread syntax - Official documentation about the spread operator in JavaScript
• MDN - Falsy values - Explanation of falsy values in JavaScript
• MDN - Logical OR operator - Documentation for the logical OR operator
• MDN - Nullish coalescing operator - Documentation for the nullish coalescing operator