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SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**'

The JavaScript exception "unparenthesized unary expression can't appear on the left-hand side of '**'" occurs when a unary operator (one of typeof, void, delete, await, !, ~, +, -) is used on the left operand of the exponentiation operator without parentheses.

Message

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)

Error type

What went wrong?

You likely wrote something like this:

js
-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.

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

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

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

Examples

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

js
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:

js
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.

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Unparenthesized_unary_expr_lhs_exponentiation