kuzzle/array-foreach #

Enforce for..of loops over Array.forEach.


Package	`eslint-plugin-kuzzle`
Type	suggestion
Severity in `configs.default`	`warn`
Fixable	no
Requires type information	no

Rationale #

Array.prototype.forEach takes a callback, and a callback cannot await, cannot break, cannot continue, and cannot return out of the enclosing function. In backend code that is almost always what you end up needing:

// The awaits resolve inside the callbacks, so `done()` runs before a single
// document has been indexed.
documents.forEach(async (document) => {
  await index(document);
});

done();

for..of has none of those problems, reads the same, and is faster on large arrays because there is no per-element function call.

Incorrect #

users.forEach((user) => {
  context.log.info(user.name);
});

Object.keys(config).forEach((key) => register(key));

Correct #

for (const user of users) {
  context.log.info(user.name);
}

for (const key of Object.keys(config)) {
  register(key);
}

Awaiting now works, and so does stopping early:

for (const document of documents) {
  if (!document.active) {
    continue;
  }

  await index(document);
}

await done();

Chained calls #

The rule reports the forEach call, wherever it sits in a chain. Collapsing the chain into one loop is usually the point:

// Reported — and iterates three times
users
  .filter((user) => user.active)
  .map((user) => user.name)
  .forEach(send);

// Once
for (const user of users) {
  if (user.active) {
    send(user.name);
  }
}

Known limitation #

The rule matches any call expression whose callee property is named forEach, whatever the receiver — it has no type information, so it cannot tell an array from something else. A Map, a Set, a NodeList, Headers or a third-party collection whose only iteration API is forEach is reported too.

That is why it is a warn and not an error. Silence it where the callback form is the right one:

// eslint-disable-next-line kuzzle/array-foreach -- Headers only exposes forEach
headers.forEach((value, name) => request.setHeader(name, value));

Configuration #

The rule takes no options.

// eslint.config.mjs
export default [
  ...kuzzle.configs.default,
  {
    rules: {
      // Promote it once the codebase is clean
      'kuzzle/array-foreach': 'error',
      // …or opt out entirely
      // 'kuzzle/array-foreach': 'off',
    },
  },
];