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The in operator is pretty useful. It allows us to check if an object has a property.

This operator returns true if the first operand is a property of the object passed on the right, or a property of one of its ancestors in its prototype chain.

Otherwise it returns false.

Example:

class Car {
  constructor() {
    this.wheels = 4
  }
}
class Fiesta extends Car {
  constructor() {
    super()
    this.brand = 'Ford'
  }
}

const myCar = new Fiesta()
'brand' in myCar //true
'wheels' in myCar //true

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