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useGuardForIn

Diagnostic Category: lint/nursery/useGuardForIn

Since: v1.9.4

Sources:

Require for-in loops to include an if statement.

Looping over objects with a for-in loop will include properties inherited through the prototype chain. This behavior can lead to unexpected items in your for loop.

For codebases that do not support ES2022, Object.prototype.hasOwnProperty.call(foo, key) can be used as a check that the property is not inherited.

For codebases that do support ES2022, Object.hasOwn(foo, key) can be used as a shorter and more reliable alternative.

Examples

Section titled Examples

Invalid

Section titled Invalid
for (key in foo) {
  doSomething(key);
}
code-block.js:1:1 lint/nursery/useGuardForIn ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ The body of a for-in should be wrapped in an if statement.
  
  > 1 │ for (key in foo) {
      │ ^^^^^^^^^^^^^^^^^^
  > 2 │   doSomething(key);
  > 3 │ }
      │ ^
    4 │ 
  
  ℹ Looping over the object with for-in loop  will include properties that are inherited through the prototype chain, the behaviour can lead to some unexpected items in your loop.
  
  ℹ To resolve this issue, add an if statement like if (Object.hasOwn(foo, key)) {...} to filter out the extraneous properties.

Valid

Section titled Valid
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);
  }
}
Section titled Related links