nullish

Nullish refers to values that are null or undefined in programming, most often in the context of JavaScript. A value is considered nullish when it is either null or undefined, signaling the absence of a value or an uninitialized variable. The concept helps distinguish these cases from other kinds of “falsy” values that can also lead to incorrect assumptions about presence, such as 0, "", or false.

Nullish is distinct from falsy values. In JavaScript, 0, "", false, and NaN are falsy, meaning they coerce to false in boolean contexts, but they are not nullish. This distinction matters for defaulting and value selection, because nullish checks specifically target only null or undefined, avoiding unintended treatment of other falsy values as missing.

A common usage pattern involves the nullish coalescing operator ??, which returns the right-hand operand when the left-hand operand is nullish. For example, let x = value ?? defaultValue; If value is null or undefined, x becomes defaultValue; otherwise x takes the value of value. This operator is often used in conjunction with optional chaining (?.) to safely access nested properties and then apply a default when necessary, as in let name = user?.profile?.name ?? 'Anonymous'.

In broader programming contexts, the idea of nullish values appears in languages with explicit null handling, such as Kotlin, C#, and TypeScript's strict null checks. The term highlights a practical approach to dealing with missing or unknown data without conflating it with other falsey conditions.