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The logical nullish assignment (??=) operator evaluates the left-hand operand and assigns the right-hand operand to it strictly if the left-hand operand is a nullish value (null or undefined). 
LeftHandSideExpression ??= AssignmentExpression

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Mechanics and Evaluation

The operator relies on short-circuit evaluation. The exact execution steps are:
  1. The LeftHandSideExpression is evaluated.
  2. If the result is strictly null or undefined, the AssignmentExpression is evaluated and its result is assigned to the LeftHandSideExpression.
  3. If the result is any other value (including non-nullish falsy values like 0, "", false, or NaN), the operator short-circuits. The AssignmentExpression is never evaluated, and no assignment operation occurs.

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Logical Equivalence

The ??= operator is logically equivalent to the following expression: 
x ?? (x = y)
It is a common misconception to equate x ??= y with x = x ?? y. While the resulting value of x is the same, the underlying mechanics differ significantly. The expression x = x ?? y will always perform an assignment operation, which will trigger object setters or proxy traps on the left-hand side. The ??= operator guarantees that the assignment (and any associated side effects) only occurs if the left-hand side is actually nullish.

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Behavior Examples

The following examples demonstrate the operator’s strict adherence to nullish checks, ignoring standard falsy values: 
let valNull = null;
valNull ??= 10; 
// Result: 10 (LHS was null)

let valUndefined = undefined;
valUndefined ??= 20; 
// Result: 20 (LHS was undefined)

let valZero = 0;
valZero ??= 30; 
// Result: 0 (LHS was falsy, but not nullish)

let valEmptyString = "";
valEmptyString ??= "text"; 
// Result: "" (LHS was falsy, but not nullish)

let valFalse = false;
valFalse ??= true; 
// Result: false (LHS was falsy, but not nullish)

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Short-Circuit Demonstration

Because the operator short-circuits, the right-hand side expression is completely ignored if the left-hand side is already initialized with a non-nullish value. 
let initializedVariable = 5;

// The function getFallbackValue() is never called
initializedVariable ??= getFallbackValue();
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