eslint/guard-for-in Style

What it does

Require for-in loops to include an if statement.

Why is this bad?

Looping over objects with a for in loop will include properties that are inherited through the prototype chain. Using a for in loop without filtering the results in the loop can lead to unexpected items in your for loop which can then lead to unexpected behaviour.

Examples

Examples of incorrect code for this rule:

for (key in foo) {
  doSomething(key);
}

Examples of correct code for this rule:

for (key in foo) {
  if (Object.hasOwn(foo, key)) {
    doSomething(key);
  }
}

for (key in foo) {
  if (Object.prototype.hasOwnProperty.call(foo, key)) {
    doSomething(key);
  }
}

for (key in foo) {
  if ({}.hasOwnProperty.call(foo, key)) {
    doSomething(key);
  }
}

How to use

To enable this rule in the CLI or using the config file, you can use:

CLI

oxlint --deny guard-for-in

Config (.oxlintrc.json)

{
  "rules": {
    "guard-for-in": "error"
  }
}

References

• Rule Source