JavaScript exponentiation unary operator design decision

问题

So I was fooling around with the new exponentiation operator and I discovered you cannot put a unary operator immediately before the base number.

let result = -2 ** 2; // syntax error
let result = -(2 ** 2); // -4
let x = 3;
let result = --x ** 2; // 4

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.

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.

I understand this was made an error by design. I don't understand this design decision. Who's really going to be surprised that -x ** 2 is negative? This follows not only other mainstream programming languages but a mathematical notation that has been in common use for hundreds of years and is taught to every high school algebra student.

In Javascript '1'+ 2 is '12' and '1'-2 is -1 but -1**2 raises an error because it could be ambiguous? Help me understand this design decision.

回答1:

I don't understand this design decision.

Read more about it at https://esdiscuss.org/topic/exponentiation-operator-precedence, https://esdiscuss.org/topic/power-operator-why-does-2-3-throws, https://github.com/rwaldron/tc39-notes/blob/master/es7/2015-09/sept-23.md#exponentiation-operator and https://github.com/rwaldron/tc39-notes/blob/master/es7/2015-09/sept-24.md#exponentiation-operator.

Who's really going to be surprised that -x ** 2 is negative?

Enough people to matter. Some relevant quotes from the above resources:

• "making ** bind tighter than unary operators would break x**-2. And making it sometimes tighter and sometimes looser would be too confusing and lead to other opportunities for precedence inversion." - Waldemar Horwat
• "Given the conflict between the history of ** in other languages, [and] the general pattern that unary binds tighter than binary, any solution at this point will confuse many people." - Mark S. Miller
• "acknowledge the prospect of significant whitespace: -x**2 === -(x ** 2) and -x ** 2 === (-x) ** 2" - Alexander Jones
• "The problem is, however rare unary minus before an exponentiation expression may be, the lack of superscript-with-smaller-font sugests that - binds tighter than **. And indeed apart from dot (a special form whose right operand must be a lexical identifier-name) and square brackets (which isn't an infix operator per se), unary operators bind tighter than binary in JS as in C and other C-derived languages." - Brendan Eich
• "For math it seems obvious that -52. But for -5 ** 2, because of the whitespace around the infix operator. Even without space, - seems to be part of the literal." - Dave Herman
• [Regarding programming language precedence], "effectively zero people have an intutition about this from other languages. Agree people have an itutition that ** is the exponentiation operator. But people usually try to avoid dark corners so they never develop an intuition for negative bases." - Dave Herman

In Javascript '1'+ 2 is '12' and '1'-2 is -1 but -1**2 raises an error because it could be ambiguous?

Well they put considerably more effort in the design of extensions to the language today :-) It's the best solution that they could reach consensus for.

来源：https://stackoverflow.com/questions/47068812/javascript-exponentiation-unary-operator-design-decision