JavaScript Optional Chaining '?.'

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The optional chaining ?. is a safe way to access nested object properties, even if an intermediate property doesn't exist.

Let's assume each user is required to fill an online form of name and address, but users mostly ignored the address input field to submit the form.

The submitted form of one of the user is modeled as an object as shown below:

const user = { name: 'Bello' }

console.log(user, user.address); // { name: 'Bello' } undefined

The user.address returns undefined because it is not filled by the user.

Let's also assume each address object must have a street. In such a case, we get an error. For deeply nested objects, properties, we get an error.

See example below:

const user = { name: 'Bello' }

console.log(user, user.address.street); 
// TypeError: Cannot read property 'street' of undefined

What if we don't want that much error but undefined for objects that don't exist.

See the example below:

const user = { name: 'Bello' }

console.log(user, user.address ? user.address.street : undefined);
// { name: 'Bello' } undefined

What if we have another nested object called name as shown below:

const user = { name: 'Bello' }

console.log(
  user, 
  user.address ? user.address.street ? user.address.street.name 
  : null : null
  );
// { name: 'Bello' } null

The code snippet above is the same as below:

const user = { name: 'Bello' }

console.log(
user, 
user.address && user.address.street && user.address.street.name
);
// { name: 'Bello' } undefined

We do not see the long string of the error in the console, but there is one issue we need to tackle - the repetitive nesting obj.obj1...objN or obj.obj1...objN.propN.

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Optional chaining solves the repetitive object nesting issue. The Optional chaining above was released in ECMAScript2020.

See the example below:

const user = {name: 'Bello'};

document.write(user, user?.address?.street?.name); 
// [object Object]undefined

Note: There must be a variable first like user for the example above to work.

If you are getting an error, an older JavaScript engine is used by your browser. You need to upgrade the browser.

Edit on Stackblitz

Let's make the user object empty below:

const user = { };

document.write(user?.address?.street?.name); 
// undefined

The ?. in an expression can also short-circuit. It is like the logical && operator. It finds the first falsy value from left to right and returns it (ignoring any other values after it).

The optional chaining ?. is not an operator, but a special syntax construct

See the examples below:

const user = null;

document.write( user?.address ); // undefined
document.write( user?.address.street ); // undefined

Recall null == undefined

const user = null;
const x = 0;

document.write?.increaseBy1(x++); 
// user value, null is falsy => .increaseBy1(x++) ignored

document.write(x); // 0

The construct, ?. also works on functions.

Syntax:

functionName?.()

See the example below:

const userAdmin = {
  admin() {
    document.write("I am admin");
  }
};

const userGuest = { };

userAdmin.admin?.(); // I am admin

userGuest.admin?.(); // no output, no error

There's also another optional chaining syntax for retrieving object keys values.

Syntax:

object?.[key]

See the example below:

const key = "firstName";

const user1 = {
  firstName: "Osagie"
};

let user2 = null;

document.write( user1?.[key] ); // Osagie
document.write( user2?.[key] ); // undefined

Also we can use ?. with delete:

delete user?.name; // ReferenceError: user is not defined

We can use ?. for safe reading and deleting, but not writing.

See the example below:

const user = null;

user?.name = "John"; // error

Happy coding

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Comments

[Tanvir Shaikh]

When was this introduced ?

[Bello Osagie]

2020