Nullish coalescing operator (??)

The nullish coalescing operator can be seen as a special case of the logical OR (||) operator. The latter returns the right-hand side operand if the left operand is any falsy value, not only null or undefined. In other words, if you use || to provide some default value to another variable foo, you may encounter unexpected behaviors if you consider some falsy values as usable (e.g., '' or 0). See below for more examples.

The nullish coalescing operator has the fifth-lowest operator precedence, directly lower than || and directly higher than the conditional (ternary) operator.

It is not possible to combine both the AND (&&) and OR operators (||) directly with ??. A syntax error will be thrown in such cases.

null || undefined ?? "foo"; // raises a SyntaxError
true && undefined ?? "foo"; // raises a SyntaxError

Instead, provide parenthesis to explicitly indicate precedence:

(null || undefined) ?? "foo"; // returns "foo"

In this example, we will provide default values but keep values other than null or undefined.

const nullValue = null;
const emptyText = ""; // falsy
const someNumber = 42;

const valA = nullValue ?? "default for A";
const valB = emptyText ?? "default for B";
const valC = someNumber ?? 0;

console.log(valA); // "default for A"
console.log(valB); // "" (as the empty string is not null or undefined)
console.log(valC); // 42

Earlier, when one wanted to assign a default value to a variable, a common pattern was to use the logical OR operator (||):

let foo;

// foo is never assigned any value so it is still undefined
const someDummyText = foo || "Hello!";

However, due to || being a boolean logical operator, the left-hand-side operand was coerced to a boolean for the evaluation and any falsy value (including 0, '', NaN, false, etc.) was not returned. This behavior may cause unexpected consequences if you consider 0, '', or NaN as valid values.

const count = 0;
const text = "";

const qty = count || 42;
const message = text || "hi!";
console.log(qty); // 42 and not 0
console.log(message); // "hi!" and not ""

The nullish coalescing operator avoids this pitfall by only returning the second operand when the first one evaluates to either null or undefined (but no other falsy values):

const myText = ""; // An empty string (which is also a falsy value)

const notFalsyText = myText || "Hello world";
console.log(notFalsyText); // Hello world

const preservingFalsy = myText ?? "Hi neighborhood";
console.log(preservingFalsy); // '' (as myText is neither undefined nor null)

Like the 'OR' and 'AND' logical operators, the right-hand side expression is not evaluated if the left-hand side proves to be neither null nor undefined.

function a() {
  console.log("a was called");
  return undefined;
}
function b() {
  console.log("b was called");
  return false;
}
function c() {
  console.log("c was called");
  return "foo";
}

console.log(a() ?? c());
// Logs "a was called" then "c was called" and then "foo"
// as a() returned undefined so both expressions are evaluated

console.log(b() ?? c());
// Logs "b was called" then "false"
// as b() returned false (and not null or undefined), the right
// hand side expression was not evaluated

The nullish coalescing operator treats undefined and null as specific values. So does the optional chaining operator (?.), which is useful to access a property of an object which may be null or undefined. Combining them, you can safely access a property of an object which may be nullish and provide a default value if it is.

const foo = { someFooProp: "hi" };

console.log(foo.someFooProp?.toUpperCase() ?? "not available"); // "HI"
console.log(foo.someBarProp?.toUpperCase() ?? "not available"); // "not available"

• Nullish coalescing assignment (??=)
• Optional chaining (?.)
• Logical OR (||)
• Default parameters