Optional Chaining in Javascript

What is Optional Chaining in Javascript?

Optional Chaining is a new feature of the Javascript language introduced in 2020 that allows for in-line checking of nested object properties. It's a very concise and easy-to-write method that also elegantly allows a program to fail. It avoids the dreaded cannot read property of undefined error message when its tries to access a property of an object that does not exist.

Optional chaining is also known as safe navigation.

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Given an object, animals, how would you assign and access a nested value of name in dog?

const animals = {
    cat: {
        name: "marge",
        age: 2,
    },
    dog: {
        name: "bob",
        age: 6,
    },
}

Without optional chaining you would first check to see if dog is a property of animals with a ternary expression:

const name = animals.dog ? animals.dog.name : undefined

This use of a ternary is a valid but a little too lengthy of a way to check for properties as you drill into a object.

With optional chaining, this can be done very efficiently:

const name = animals.dog?.name

Optional chaining syntax is using the ? to check if dog exists as a property of animals. If it isn't, it will return undefined and the object chaining short circuits so that name is never called. If dog does exist as a property of animals, then the program will proceed and attempt to get dog's property of name.

At this point optional chaining does not care if name exists as a property -- and in fact there is not a way to optional chain the end of object drilling. This syntax is incorrect:

const name = animals.dog?.name?.

But this syntax is fine:

const name = animals?.dog

In the following example you can see how elegant it is to sneak in a ? into dot notation chaining, while also ensuring that the program will fail gracefully by returning undefined instead of throwing an error.

const dogName = animals.dog.name

const catName = animals.cat?.name

const duckName = animals.duck?.name

const dogWeight = animals.dog?.weight

console.table({
    dogName: dogName,
    catName: catName,
    duckName: duckName,
    dogWeight: dogWeight,
})

This is what will be logged:

index	Value
dogName	'bob'
catName	'marge'
duckName	undefined
dogWeight	undefined

Beyond searching properties of an object, optional chaining can also be used to check the truthy-ness of object's expressions, array indexes, and function arguemnts.

obj.val?.prop
obj.val?.[expr]
obj.arr?.[index]
obj.func?.(args)

The quirky thing to note is that optional chaining requires both symbols ? and . even in situations when dot notation is not used, such as in accessing an array with an index.

Optional chaining (a.k.a. safe navigation) may have slightly different syntax in different languages. In Ruby, instead of ?. the language uses &. for interesting reasons. In many languages, though, something similar to ? is used.

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References:

MDN

FreeCodeCamp