SyntaxError: Unary operator used immediately before exponentiation expression. Parenthesis must be used to disambiguate operator precedence (V8-based)
SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**' (Firefox)
SyntaxError: Unexpected token '**'. Ambiguous unary expression in the left hand side of the exponentiation expression; parentheses must be used to disambiguate the expression. (Safari)

You likely wrote something like this:

-a ** b

Whether it should be evaluated as (-a) ** b or -(a ** b) is ambiguous. In mathematics, -x² means -(x ** 2) — and that's how many languages, including Python, Haskell, and PHP, handle it. But making the unary minus operator take precedence over ** breaks symmetry with a ** -b, which is unambiguously a ** (-b). Therefore, the language forbids this syntax and requires you to parenthesize either side to resolve the ambiguity.

(-a) ** b
-(a ** b)

Other unary operators cannot be the left-hand side of exponentiation either.

await a ** b
!a ** b
+a ** b
~a ** b

When writing complex math expressions involving exponentiation, you may write something like this:

function taylorSin(x) {
  return (n) => (-1 ** n * x ** (2 * n + 1)) / factorial(2 * n + 1);
  // SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**'
}

However, the -1 ** n part is illegal in JavaScript. Instead, parenthesize the left operand:

function taylorSin(x) {
  return (n) => ((-1) ** n * x ** (2 * n + 1)) / factorial(2 * n + 1);
}

This also makes the code's intent much clearer to other readers.

• Exponentiation (**)
• Operator precedence
• Original discussion of exponentiation operator precedence on esdiscuss.org