The nullish coalescing operator in JavaScript is defined as two question marks ??. The left-hand side of the operator is returned as long as the right-hand side is neither null nor undefined and vice-versa.


Right-hand side:

const example1 = 'Hello' ?? null //Result: Hello

const example2 = 'Hello' ?? undefined //Result: Hello

Left-hand side:

const example3 = null ?? 'Hello' //Result: Hello

const example4 = undefined ?? 'Hello' //Result: Hello


The rule above holds true except when you are dealing with a falsy value.

Difference between nullish coalescing and OR (||)

Like nullish coalescing, OR (||) is a logical operator. The difference between them is that logical OR evaluates to true iff (if and only if) “one or more of its operands is true.” When used with Boolean values, a Boolean value is returned. However, when used with non-Boolean values, non-Boolean values are returned.[1]


const a = 1;
const b = 2;
const c = 3;
const d = 4;

console.log(a > b || c > d); //(1 > 2 || 3 > 4) Result: false

Using const b = 2 from the previous example:

const string = "";

console.log(string || b); //Result: 2

[1] Logical OR (||)