Problem generating exponentiation operator with unary operator applied to base

[stonegray]

I believe I've found an error in how code is generated when a UnaryExpression is used as the base in an ExponentiationExpression:

(-2)**4

From MDN:

In JavaScript, it is impossible to write an ambiguous exponentiation expression, i.e. you cannot put a unary operator (+/-/~/!/delete/void/typeof) immediately before the base number.

I am using the following AST. I believe this is the correct AST to produce the desired outpput as acorn, cherow, and meriyah all produce the same output for this code. I was not able to test in esprima or espree as this code crashes them.

{
    "type": "Program",
    "sourceType": "script",
    "body": [
        {
            "type": "ExpressionStatement",
            "expression": {
                "type": "BinaryExpression",
                "left": {
                    "type": "UnaryExpression",
                    "operator": "+",
                    "argument": {
                        "type": "Literal",
                        "value": 2
                    },
                    "prefix": true
                },
                "right": {
                    "type": "Literal",
                    "value": 4
                },
                "operator": "**"
            }
        }
    ]
}

Expected behavior

Code should be generated with parentheses around the base when a unary operator is applied.

astring.generate(acorn.parse('(+2)**4'))

// Expected Output:
(+2)**4;

Actual behavior

No parentheses are added, leading to invalid code that throws an error when executed.

astring.generate(acorn.parse('(+2)**4'))

// Observed Output:
+2**4;

Error:

+2**4
^^^^

SyntaxError: Unary operator used immediately before exponentiation expression. Parenthesis must be used to disambiguate operator precedence

---

See also: jquery/esprima#1981

[davidbonnet]

Thanks @stonegray for reporting this. It is fixed v1.4.1.