1st October 2020

Nullish coalescing operator '??'

A recent addition
This is a recent addition to the language. Old browsers may need polyfills.

Here, in this article, we’ll say that an expression is “defined” when it’s neither null nor undefined.

The nullish coalescing operator is written as two question marks ??.

The result of a ?? b is:

  • if a is defined, then a,
  • if a isn’t defined, then b.

In other words, ?? returns the first argument if it’s not null/undefined. Otherwise, the second one.

The nullish coalescing operator isn’t anything completely new. It’s just a nice syntax to get the first “defined” value of the two.

We can rewrite result = a ?? b using the operators that we already know, like this:

result = (a !== null && a !== undefined) ? a : b;

The common use case for ?? is to provide a default value for a potentially undefined variable.

For example, here we show Anonymous if user isn’t defined:

let user;

alert(user ?? "Anonymous"); // Anonymous

Of course, if user had any value except null/undefined, then we would see it instead:

let user = "John";

alert(user ?? "Anonymous"); // John

We can also use a sequence of ?? to select the first value from a list that isn’t null/undefined.

Let’s say we have a user’s data in variables firstName, lastName or nickName. All of them may be undefined, if the user decided not to enter a value.

We’d like to display the user name using one of these variables, or show “Anonymous” if all of them are undefined.

Let’s use the ?? operator for that:

let firstName = null;
let lastName = null;
let nickName = "Supercoder";

// shows the first defined value:
alert(firstName ?? lastName ?? nickName ?? "Anonymous"); // Supercoder

Comparison with ||

The OR || operator can be used in the same way as ??, as it was described in the previous chapter.

For example, in the code above we could replace ?? with || and still get the same result:

let firstName = null;
let lastName = null;
let nickName = "Supercoder";

// shows the first truthy value:
alert(firstName || lastName || nickName || "Anonymous"); // Supercoder

The OR || operator exists since the beginning of JavaScript, so developers were using it for such purposes for a long time.

On the other hand, the nullish coalescing operator ?? was added to JavaScript only recently, and the reason for that was that people weren’t quite happy with ||.

The important difference between them is that:

  • || returns the first truthy value.
  • ?? returns the first defined value.

In other words, || doesn’t distinguish between false, 0, an empty string "" and null/undefined. They are all the same – falsy values. If any of these is the first argument of ||, then we’ll get the second argument as the result.

In practice though, we may want to use default value only when the variable is null/undefined. That is, when the value is really unknown/not set.

For example, consider this:

let height = 0;

alert(height || 100); // 100
alert(height ?? 100); // 0
  • The height || 100 checks height for being a falsy value, and it really is.
    • so the result is the second argument, 100.
  • The height ?? 100 checks height for being null/undefined, and it’s not,
    • so the result is height “as is”, that is 0.

If the zero height is a valid value, that shouldn’t be replaced with the default, then ?? does just the right thing.

Precedence

The precedence of the ?? operator is rather low: 5 in the MDN table. So ?? is evaluated before = and ?, but after most other operations, such as +, *.

So if we’d like to choose a value with ?? in an expression with other operators, consider adding parentheses:

let height = null;
let width = null;

// important: use parentheses
let area = (height ?? 100) * (width ?? 50);

alert(area); // 5000

Otherwise, if we omit parentheses, then as * has the higher precedence than ??, it would execute first, leading to incorrect results.

// without parentheses
let area = height ?? 100 * width ?? 50;

// ...works the same as this (probably not what we want):
let area = height ?? (100 * width) ?? 50;

Using ?? with && or ||

Due to safety reasons, JavaScript forbids using ?? together with && and || operators, unless the precedence is explicitly specified with parentheses.

The code below triggers a syntax error:

let x = 1 && 2 ?? 3; // Syntax error

The limitation is surely debatable, but it was added to the language specification with the purpose to avoid programming mistakes, when people start to switch to ?? from ||.

Use explicit parentheses to work around it:

let x = (1 && 2) ?? 3; // Works

alert(x); // 2

Summary

  • The nullish coalescing operator ?? provides a short way to choose the first “defined” value from a list.

    It’s used to assign default values to variables:

    // set height=100, if height is null or undefined
    height = height ?? 100;
  • The operator ?? has a very low precedence, only a bit higher than ? and =, so consider adding parentheses when using it in an expression.

  • It’s forbidden to use it with || or && without explicit parentheses.

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