** is the exponentiation operator and is the equivalent of Math.pow.
It was introduced in ECMAScript 2016 (ES7).
** operator is a valid operator in ES7. It holds the same meaning as Math.pow(x,y) for example, 2**3 is same as Math.pow(2,3)
Example
console.log(4 ** 4); // 256
console.log(8 ** 4); // 4096
console.log(10 ** 4); // 10000