forInGuards

Reports for-in loops without an if statement to filter inherited properties.

✅ This rule is included in the ts preset.

Looping over objects with a for-in loop will include properties inherited through the prototype chain. This can lead to unexpected items being iterated over when the object inherits from a non-standard prototype.

for (const 
const key: string
key in 
const object: Record<string, unknown>
object) {
  
const doSomething: (key: string) => void
doSomething(
const key: string
key);
}

for (const 
const key: string
key in 
const object: Record<string, unknown>
object) {
  
const doSomething: (key: string) => void
doSomething(
const key: string
key);
}

for (const 
const key: string
key in 
const object: Record<string, unknown>
object) {
  if (
const condition: boolean
condition) {
    
const doSomething: (key: string) => void
doSomething(
const key: string
key);
  }
  
const doSomethingElse: (key: string) => void
doSomethingElse(
const key: string
key);
}

for (const 
const key: string
key in 
const object: Record<string, unknown>
object) {
  if (
var Object: ObjectConstructor
Provides functionality common to all JavaScript objects.
Object.
ObjectConstructor.hasOwn(o: object, v: PropertyKey): boolean
Determines whether an object has a property with the specified name.
@param ― o An object.
@param ― v A property name.
hasOwn(
const object: Record<string, unknown>
object, 
const key: string
key)) {
    
const doSomething: (key: string) => void
doSomething(
const key: string
key);
  }
}

for (const 
const key: string
key in 
const object: Record<string, unknown>
object) {
  if (!
var Object: ObjectConstructor
Provides functionality common to all JavaScript objects.
Object.
ObjectConstructor.hasOwn(o: object, v: PropertyKey): boolean
Determines whether an object has a property with the specified name.
@param ― o An object.
@param ― v A property name.
hasOwn(
const object: Record<string, unknown>
object, 
const key: string
key)) {
    continue;
  }
  
const doSomething: (key: string) => void
doSomething(
const key: string
key);
}

for (const 
const key: string
key in 
const object: Record<string, unknown>
object) {
  if (
var Object: ObjectConstructor
Provides functionality common to all JavaScript objects.
Object.
ObjectConstructor.prototype: Object
A reference to the prototype for a class of objects.
prototype.
Object.hasOwnProperty(v: PropertyKey): boolean
Determines whether an object has a property with the specified name.
@param ― v A property name.
hasOwnProperty.
CallableFunction.call<Record<string, unknown>, [string], boolean>(this: (this: Record<string, unknown>, args_0: string) => boolean, thisArg: Record<string, unknown>, args_0: string): boolean
Calls the function with the specified object as the this value and the specified rest arguments as the arguments.
@param ― thisArg The object to be used as the this object.
@param ― args Argument values to be passed to the function.
call(
const object: Record<string, unknown>
object, 
const key: string
key)) {
    
const doSomething: (key: string) => void
doSomething(
const key: string
key);
  }
}

for (const 
const key: string
key in 
const object: Record<string, unknown>
object);

for (const 
const key: string
key in 
const object: Record<string, unknown>
object) {
}

Options

This rule is not configurable.

When Not To Use It

If you are certain that the objects you iterate over do not have inherited properties from custom prototypes, you might choose to disable this rule. This is common when iterating over plain objects created with object literals or Object.create(null).

Equivalents in Other Linters

Made with ❤️‍🔥 around the world by the Flint team and contributors.

forInGuards

Examples

Options

When Not To Use It

Further Reading

Equivalents in Other Linters