The nullish coalescing (??) operator is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.

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)