How to use the ?. operator in Javascript

Because in Javascript objects can be very complex and have unpredictable structures, sometimes is hard to access their properties. There's a real possibility that some nested objects or properties might not exist so when trying to use them, we run into errors. Optional chaining, as a part of ES2020, changes and simplifies the way properties are accessed from deep objects structures. It is also available in TypeScript, starting from version 3.7.

The optional chaining operator (?.) permits reading the value of a property located deep within a chain of connected objects without having to expressly validate that each reference in the chain is valid. The ?. operator functions similarly to the . chaining operator, except that instead of causing an error if a reference is nullish (null or undefined), the expression short-circuits with a return value of undefined. When used with function calls, it returns undefined if the given function does not exist.

Without the optional chaining operator, if we want to check for properties inside a deeply nested object, for example, we would need to first check if every reference exists, otherwise we risk getting into errors.

let person = {
    name: "Ben",
    age: 35,
    bankDetails: {
        IBAN: 781265489634,
        accountNumber: 5689245522221

    }
}

console.log(person.hobbies.fishing); // Uncaught TypeError: Cannot read property 'sum' of undefined

We get an error because we are trying to read the fishing property on the hobbies object but the person object has no such nested object inside it. A workaround for this would be to write the code like so, using the && operator:

let person = {
    name: "Ben",
    age: 35,
    bankDetails: {
        IBAN: 781265489634,
        accountNumber: 5689245522221

    }
}

console.log(person && person.hobbies && person.hobbies.fishing); // undefined

The property still doesn't exist, but instead of a TypeError exception we not get back "undefined". Adding the && operator makes sure that all properties we're trying to check exist but it seems like a lot of work to always write it and it's hard to read too.

This is how the same code looks like with the chaining operator.

let person = {
    name: "Ben",
    age: 35,
    bankDetails: {
        IBAN: 781265489634,
        accountNumber: 5689245522221

    }
}

console.log(person?.hobbies?.fishing);// undefined

The code looks much shorter and cleaner. The way the optional chaining operator works is that it stops the evaluation and returns undefined if the part before ?. is null or undefined.

A small warning: not all browsers support this operator so make sure you don't need to use polyfills to avoid issues. You can check the compatibility here.

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