Check if a property exists in the object or array: `in`

Check if a property exists in the object or array: in

If you have an object and you want to check if a property is present you can use the in operator.

const fruits = {
  strawberry: 'Strawberry',
  kiwi: 'Kiwi',
  lemon: 'Lemon'
}
// for checking the presence of an property you can use in operator
if ('kiwi' in fruits) {
  console.log('Kiwi is present')
}
// you can use in the short form
console.log('kiwi' in fruits ? 'Yes! Kiwi' : 'ther is no kiwi here')

I think the in operator is one of the best ways to check the presence of a property because, for example, if you want to check if the value is undefined, you can’t distinguish the cases if the property doesn’t exist or the property exists and has an undefined value:

const fruits = {
  strawberry: 'Strawberry',
  kiwi: 'Kiwi',
  lemon: 'Lemon'
}

fruits.something = undefined
if (fruits.somethingelse === undefined) {
    console.log('Somethingelse property does not exist')
}
if (fruits.something === undefined) {
  console.log('Something property exists but the value is undefined')
}

The in operator works also with the arrays. With the arrays, you can check the presence of the numeric index:

const fruitsArray = []
fruitsArray.push('Strawberry')
fruitsArray.push('Kiwi')
fruitsArray.push('Lemon')
console.dir(fruitsArray)
console.log(2 in fruitsArray) // true
console.log(3 in fruitsArray) // false