SyntaxError: unparenthesized unary expression can't appear on the left-hand side of ''
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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#
{{jsxref("SyntaxError")}}
What went wrong?#
You likely wrote something like this:
```js-nolint example-bad -a ** b
Whether it should be evaluated as `(-a) ** b` or `-(a ** b)` is ambiguous. In mathematics, -x<sup>2</sup> 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-nolint example-good
(-a) ** b
-(a ** b)
Other unary operators cannot be the left-hand side of exponentiation either.
```js-nolint example-bad await a ** b !a ** b +a ** b ~a ** b
## Examples
When writing complex math expressions involving exponentiation, you may write something like this:
```js-nolint example-bad
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 example-good
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.