JavaScript Code Review

Includes all guidelines from code-review skill, plus JavaScript-specific rules.

Prefer Functional Over OO

The reviewer SHOULD:

• Prefer pure functions and data transforms over classes when state is not required.
• Encourage small, composable utilities instead of deep inheritance trees.

The reviewer MUST:

• Flag new classes that only wrap stateless helpers or act as namespaces.

Acceptable:

const formatUser = (user) => `${user.firstName} ${user.lastName}`;

Discouraged:

class UserFormatter {
  format(user) {
    return `${user.firstName} ${user.lastName}`;
  }
}

Use

switch

for Enum-like Values

The reviewer MUST:

• Flag

  if/else if

  chains that branch on the same enum-like value when a

  switch

  is clearer.

Preferred:

switch (status) {
  case "idle":
    return renderIdle();
  case "running":
    return renderRunning();
  case "failed":
    return renderFailed();
  default:
    return assertNever(status);
}

Discouraged:

if (status === "idle") return renderIdle();
if (status === "running") return renderRunning();
if (status === "failed") return renderFailed();

Avoid Duplicated Code

The reviewer MUST:

• Flag duplicated logic and suggest extracting helpers or shared utilities.

The reviewer SHOULD:

• Prefer a single source of truth for calculations and formatting.

Use Modern JavaScript APIs When Available

The reviewer SHOULD:

• Prefer modern APIs like

  Object.hasOwn

  ,

  Array.prototype.at

  ,

  flatMap

  ,

  replaceAll

  ,

  URL

  ,

  AbortController

  , and

  Promise.any

  when they improve clarity.
• Confirm runtime targets or polyfills before requiring newer APIs.