Regarding

p = 0;
(p+1)++;
> ReferenceError: Invalid left-hand side expression in postfix operation

and

p = 0;
++(p+4);
> ReferenceError: Invalid left-hand side expression in prefix operation

I just got a bit of a surprise, as I expected postfix/prefix operators to be ok with working on the resolution of the expression (brackets have the highest operator precedence).

Could someone give me with a line or three to explain what is happening here?

Thanks

EDIT: Thanks for the quick responses, first answer marked as the answer. I feel I should also point people to the indepth answer from @thefourtheye below

Regarding

p = 0;
(p+1)++;
> ReferenceError: Invalid left-hand side expression in postfix operation

and

p = 0;
++(p+4);
> ReferenceError: Invalid left-hand side expression in prefix operation

I just got a bit of a surprise, as I expected postfix/prefix operators to be ok with working on the resolution of the expression (brackets have the highest operator precedence).

Could someone give me with a line or three to explain what is happening here?

Thanks

EDIT: Thanks for the quick responses, first answer marked as the answer. I feel I should also point people to the indepth answer from @thefourtheye below

• javascript

• 1 what is p here ?? a variable? – Dimag Kharab Commented Apr 2, 2014 at 6:09
• 1 first try to convert p in integer – joker Commented Apr 2, 2014 at 6:09

4 Answers 4

++ increments the value of a variable, so it is larger than before. Eg:

var x = 3;
x++;
alert(x); // will show 4

For there to be any point for this, the expression to the left of ++ must be accessible and mutable, otherwise the increment would be possible. Eg:

3++

doesn't make any sense, as 3 is a constant and can't be incremented. We don't want this to be possible:

3++;
alert(3); // outputs 4???

This is why your expression doesn't work. Ie:

var p = 2;
(p + 1)++;

has the same problem as above. (p + 1) will evaluate to 3, and ++ can't change the value of the constant 3.

You are trying to increment (), the increment/decrement operator can be apply on variable, try the sample code

(p++) + 1

OR

(++p) + 1

Remember that when you write p++, that actually gets translated to p = p + 1. The operators ++ and -- are convenience notation for incrementing/decrementing a variable for future use. But how is (p+1)++ or ++(p+4) supposed to be translated? Those sort of imply that 1 or 4 are being incremented/decremented for future use, which doesn't make sense.

When you have an expression like this

(expr)++;

These are the operations JavaScript will do internally

1. Resolve the actual object referenced by expr.

   This step is important, because you can even do something like this

   var a = {b: 1};
   ++a.b;
   a.b++;
   console.log(a.b);
   # 3
   

   Now, JavaScript has to resolve the actual object to be incremented. In this case, it will be b in a.

2. Get the value at the reference and convert that value to a Number.

   This step is also very important, because you may even have values like this

   var a = {b: '1'};
   console.log(++a.b);
   # 2
   

   JavaScript will try its best to get a number value, instead of failing immediately.

3. Increment the number.

4. Store the new vale in expr. This is step where your expression is failing.

   In your case expr is p + 1, when it is resolved the value would be just a numeral, whose value can never be changed. (You can never change the value of 1 to something else). So, after the incrementing part, when the new value has to be stored back, JavaScript doesn't find a valid reference to store it. That is why it throws this error.

   ReferenceError: Invalid left-hand side expression in postfix operation

This error message is actually thrown from internal PutValue method. The very first step goes like this

If Type(V) is not Reference, throw a ReferenceError exception.

Reference: ECMA Script 5.1 Standard Specification for Prefix Increment Operator