In a JavaScript program, the correct way to check if an object property is undefined is to use the typeof operator.

typeof returns a string that tells the type of the operand. It is used without parentheses, passing it any value you want to check:

const list = []
const count = 2

typeof list //"object"
typeof count //"number"
typeof "test" //"string"

typeof color //"undefined"

If the value is not defined, typeof returns the ‘undefined’ string.

Now suppose you have a car object, with just one property:

const car = {
  model: 'Fiesta'
}

This is how you check if the color property is defined on this object:

if (typeof car.color === 'undefined') {
  // color is undefined
}