Nullish coalescing and optional chaining in JavaScript

Nullish coalescing operator

The nullish coalescing operator (??) is a logical operator that returns the right-side operand when the operand on its left-side either evaluates to null or undefined.

Unlike to logical OR operator (||) which returns the right-hand operand if the left is a falsy value, the nullish coalescing operator does not return the right-hand operand for other falsy values like 0 and ''.

const logical_or_test = 0 || 45;
const nullish_coalescing_test = 0 ?? 45;

console.log(logical_or_test); // 45
console.log(nullish_coalescing_test); // 0

According to the tc39 proposal site, the design decisions made for the operator are:

  1. The right argument of ?? is evaluated only if needed ("short circuiting").
  2. ?? has lower precedence than ||.
  3. ?? cannot immediately contain, or be contained within, an && or || operation.
  4. The right argument is selected if the left argument is null or undefined.

No chaining with AND or OR

Chaining the nullish coalescing operator with logical AND or OR will throw a syntax error. However, wrapping the expression with a logical AND or OR in a pair of parentheses to show precedence is fine.

This throws a syntax error

0 || undefined ?? "a string";

But this is okay, because of the parentheses

(0 || undefined) ?? 'a string';

To know more about nullish coalescing, check out the documentation on MDN.

Optional chaining operator

The optional chaining operator (?.) is used to access nested properties of an object without having to check each reference for validity

It functions just as the chaining operator (.) except that it doesn't throw an error when a reference happens to be nullish (null or undefined). The expression is short-circuited with a return value of undefined when the property being accessed is nullish.

Basic usage

Assuming we have an object containing some random properties

const user = {
  name: {
    firstName: 'James',
    lastName: 'carter'
  },
  age: 24
};

To access the value of a nested property without having to validate.

var hairColor = user.hair?.color;
console.log(hairColor); // undefined

This syntax simplifies the process of accessing properties which may potentially be nullish.

Trying to access the nested color property would throw an error:

var hairColor = user.hair.color;
console.log(hairColor);
// => Uncaught TypeError: Cannot read property 'color' of undefined

Normally we would need to use the logical AND operator (&&) for validation:

var hairColor = user.hair && user.hair.color;
console.log(hairColor); // undefined

We could also use the ternary operator for validation:

var hairColor = user.hair ? user.hair.color : undefined;
console.log(hairColor); // undefined

Optional chaining with function calls

Calling a function that doesn't exist using ?. would return undefined. However, if the property name being invoked as a function exists already but not as a function, an error would be thrown.

const user = {
  name: {
    firstName: 'James',
    lastName: 'carter'
  },
  age: 24
};

// accessing a non-existent method
console.log(user.name?.getInitials?.());
// => undefined

Firstly we check if the property name exists before invoking as a function, that way if the property is nullish, we get undefined instead of the error undefined is not function.

If a property name already exists but not as a function, we would get an error:

console.log(user.name?.firstName?.());
// => Error: user.name.firstName is not a function

Check out the docs on MDN to read more on optional chaining.

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