Exponentiation assignment (**=)

The exponentiation assignment (**=) operator performs exponentiation 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 bar = 5;
bar **= 2; // 25

Other non-BigInt values are coerced to numbers:

let baz = 5;
baz **= "foo"; // NaN

let foo = 3n;
foo **= 2n; // 9n
foo **= 1; // TypeError: Cannot mix BigInt and other types, use explicit conversions

Specification
ECMAScript Language Specification # sec-assignment-operators

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