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 mon 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.

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 mon 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.

• javascript
• operator-precedence
• exponentiation
• ecmascript-2016

• 2 Writing -x ** 2 instead of - x**2 would make it ambiguous alone. – Bergi Commented Nov 2, 2017 at 6:41
• Well, I believe I was always taught at school that -x raised to the power of y produces a positive number. So, -1**2 should produce 1. At least this is what I believe was taught, and I am sufficiently confident that my belief is correct. – doubleOrt Commented Dec 9, 2017 at 16:55
• 1 Why the heck was this question downvoted ? – doubleOrt Commented Dec 9, 2017 at 17:25
• 1 @doubleOrt - Agreed, very strange (about the vote). BTW, in math, -1² is -1. It's -(1²), not (-1)². I went 'round and 'round in my head on that one and finally punted and asked the question. It's been that long since math class. But I'm also a bit embarrassed to have asked, since I routinely see things like -2³² + 1 and have no trouble interpreting that correctly (2³² = 4294967296, -4294967296 + 1 is -4294967295). – T.J. Crowder Commented Dec 5, 2018 at 15:54
• @T.J._Crowder I am actually quite ashamed of my ment now that I read it, -1**2 only takes a basic understanding of operator precedence in Math, and "(-x)" raised to the power of "y" does not always produce a positive. – doubleOrt Commented Dec 6, 2018 at 13:43

2 Answers 2

I don't understand this design decision.

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

Since we usually interpret -5² as -25, I suggest we allow -5 ** 2 (which would be -25), then we disallow 5**-2 just like we also disallow 5--2, 5+* 2, 5*/2, etc where we do not put different operators one after another.

Allowing -5**2 would not break with the math munity and their libraries where they usually write like -5^2 and 5^(-2) and 5-(-2) instead of just 5--2.