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The logical AND assignment (&&=) operator assigns the value of its right operand to its left operand only if the left operand evaluates to a truthy value. It combines logical AND (&&) short-circuiting evaluation with variable assignment.

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Syntax

leftExpr &&= rightExpr

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Mechanical Equivalence

The &&= operator is logically equivalent to the following expression: 
leftExpr && (leftExpr = rightExpr);
It is a common misconception to equate x &&= y with x = x && y. While the resulting value is often the same, the underlying execution differs significantly due to short-circuiting:
  • In x = x && y, the assignment operation always occurs, regardless of whether x is truthy or falsy.
  • In x &&= y, if x is falsy, the assignment operation is entirely skipped. This prevents unnecessary setter invocations and avoids triggering side effects associated with property assignment.

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Evaluation Rules

  1. The JavaScript engine evaluates leftExpr.
  2. It determines the boolean coercion (truthiness) of the evaluated leftExpr.
  3. If leftExpr is truthy: The engine evaluates rightExpr and assigns the resulting value to leftExpr.
  4. If leftExpr is falsy: The engine short-circuits. rightExpr is never evaluated, no assignment takes place, and leftExpr retains its original value.
(Note: Falsy values in JavaScript are false, 0, -0, 0n, "", null, undefined, and NaN. All other values are truthy.)

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Code Visualization

Truthy Evaluation (Assignment Occurs): 
let x = 5;      // 5 is truthy
x &&= 10;       // x is truthy, so 10 is assigned to x
console.log(x); // 10
Falsy Evaluation (Short-Circuiting): 
let y = 0;      // 0 is falsy
y &&= 10;       // y is falsy, assignment is skipped
console.log(y); // 0
Proof of Short-Circuiting: 
let z = null;

// The function on the right is never invoked because 'z' is falsy.
z &&= (function() {
    console.log("Right operand evaluated");
    return "new value";
})();

console.log(z); // null

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Return Value

The &&= expression returns the final value of leftExpr after the operation completes. If an assignment occurred, it returns the value of rightExpr. If it short-circuited, it returns the original falsy value of leftExpr.
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