Bitwise AND assignment (&=)

The bitwise AND assignment (&=) operator performs bitwise AND on the two operands and assigns the result to the left operand.

Try it

x &= y

x &= y is equivalent to x = x & y, except that the expression x is only evaluated once.

let a = 5;
// 5:     00000000000000000000000000000101
// 2:     00000000000000000000000000000010
a &= 2; // 0

let b = 5n;
b &= 2n; // 0n

Specification
ECMAScript Language Specification # sec-assignment-operators

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