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The nullish coalescing operator (??) is a logical operator that returns its right-hand operand when its left-hand operand evaluates strictly to a nullish value (null or undefined). For all other left-hand values—including falsy values such as 0, "" (empty string), NaN, and false—it returns the left-hand operand. Syntax 
leftExpression ?? rightExpression
Evaluation Mechanics
  • Strict Nullish Check: Unlike the logical OR operator (||), which coerces the left operand to a boolean and evaluates to the right operand for any falsy value, ?? strictly checks for the absence of a value (null or undefined).
  • Short-Circuit Evaluation: The ?? operator utilizes short-circuiting. If leftExpression evaluates to a non-nullish value, the JavaScript engine bypasses the evaluation of rightExpression entirely.
Behavioral Examples 
// Nullish values trigger the right-hand operand
const a = null ?? 'fallback';       // Evaluates to 'fallback'
const b = undefined ?? 'fallback';  // Evaluates to 'fallback'

// Falsy (but non-nullish) values return the left-hand operand
const c = 0 ?? 42;                  // Evaluates to 0
const d = "" ?? "fallback text";    // Evaluates to ""
const e = false ?? true;            // Evaluates to false
const f = NaN ?? 100;               // Evaluates to NaN
Operator Precedence and Grouping According to the ECMAScript specification, the ?? operator has lower precedence than the logical AND operator (&&), but shares the exact same precedence level as the logical OR operator (||). Both ?? and || are parsed as immediate productions of ShortCircuitExpression. Because ?? shares a precedence level with || but has no defined associativity with either || or &&, the specification strictly forbids chaining ?? directly with && or || operators without explicit grouping. Attempting to mix these operators at the same nesting level results in a SyntaxError. 
// SyntaxError: Cannot use unparenthesized `??` within logical OR/AND expressions
null || undefined ?? "foo"; 
true && undefined ?? "foo";

// Correct usage requires explicit grouping via parentheses
(null || undefined) ?? "foo"; // Evaluates to "foo"
true && (undefined ?? "foo"); // Evaluates to "foo"
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