From the documentation on 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.

The reason is also explained in that same text:

In most languages like PHP and Python and others that have an exponentiation operator (typically ^ or **), the exponentiation operator is defined to have a higher precedence than unary operators such as unary + and unary -, but there are a few exceptions. For example, in Bash the ** operator is defined to have a lower precedence than unary operators.

So to avoid confusion it was decided that the code must remove the ambiguity and explicitly put the parentheses:

(-1)**2

or:

-(1**2) 

As a side note, the binary - is not treated that way -- having lower precedence -- and so the last expression has the same result as this valid expression:

0-1**2

Exponentiation Precedence in Other Programming Languages

As already affirmed in above quote, most programming languages that have an infix exponentiation operator, give a higher precedence to that operator than to the unary minus.

Here are some other examples of programming languages that give a higher precedence to the unary minus operator:

• bc
• VBScript
• AppleScript
• COBOL
• Rexx
• Orc