Unfiltered for..in loop

Last modified: 03 December 2024

Reports unfiltered for-in loops.

The use of this construct results in processing not only own properties of an object but properties from its prototype as well. It may be unexpected in some specific cases, for example, in utility methods that copy or modify all properties or when Object's prototype may be incorrectly modified. For example, the following code will print 42 and myMethod:

Object.prototype.myMethod = function myMethod() {};
let a = { foo: 42 };
for (let i in a) {
  console.log(a[i]);
}

Suggests replacing the whole loop with a Object.keys() method or adding a hasOwnProperty() check. After applying the quick-fix the code looks as follows:

for (let i in a) {
  if (a.hasOwnProperty(i)) {
    console.log(a[i]);
  }
}

Locating this inspection

By ID

Can be used to locate inspection in e.g. Qodana configuration files, where you can quickly enable or disable it, or adjust its settings.

JSUnfilteredForInLoop
Via Settings dialog

Path to the inspection settings via IntelliJ Platform IDE Settings dialog, when you need to adjust inspection settings directly from your IDE.

Settings or Preferences | Editor | Inspections | JavaScript and TypeScript | General

Inspection options

Here you can find the description of settings available for the Unfiltered for..in loop inspection, and the reference of their default values.

Skip iterations over reference with array or object initializer

Default: Selected

Inspection Details

By default bundled with:

CLion 2024.3, GoLand 2024.3, IntelliJ IDEA 2024.3, JetBrains Rider 2024.3, PhpStorm 2024.3, PyCharm 2024.3, Qodana for .NET 2024.3, Qodana for Go 2024.3, Qodana for JS 2024.3, Qodana for JVM 2024.3, Qodana for PHP 2024.3, Qodana for Ruby 2024.3, RubyMine 2024.3, WebStorm 2024.3

Can be installed with plugin:

JavaScript and TypeScript, 243.23139