specifications - JavaScript increment unary operator (++) on strings - Stack Overflow

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I am on Chromium Version 53.0.2785.143 Built on Ubuntu, running on Ubuntu 16.04 (64-bit)

According to the ECMAScript® Language Specification, prefix increment operator is evaluated as follows:

With this in mind, I cannot explain this result:

++'1';
> Uncaught ReferenceError: Invalid left-hand side expression in prefix operation

when the following code works like a charm:

var x = '1'; 
++x;
> 2

As far as I understand, in both cases the first 3 bullet points of the second step are true, whereas for ++'1' case the fourth bullet is also true (but why?) and for the ++x case it is false, raising no error. Am I right?

PS: Firefox throws a SyntaxError: invalid increment operand instead of a ReferenceError

I am on Chromium Version 53.0.2785.143 Built on Ubuntu, running on Ubuntu 16.04 (64-bit)

According to the ECMAScript® Language Specification, prefix increment operator is evaluated as follows:

With this in mind, I cannot explain this result:

++'1';
> Uncaught ReferenceError: Invalid left-hand side expression in prefix operation

when the following code works like a charm:

var x = '1'; 
++x;
> 2

As far as I understand, in both cases the first 3 bullet points of the second step are true, whereas for ++'1' case the fourth bullet is also true (but why?) and for the ++x case it is false, raising no error. Am I right?

PS: Firefox throws a SyntaxError: invalid increment operand instead of a ReferenceError

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Share Improve this question Follow asked Oct 12, 2016 at 13:56 iulianiulian 5,84233 gold badges3232 silver badges4040 bronze badges 2

• 1 You get the same error if you do ++1, by the way – Joseph Young Commented Oct 12, 2016 at 13:58
• 1 PutValue('1', 2) throws an error because it cannot assign to a string literal. You need a variable or some other kind of Reference. – Bergi Commented Oct 12, 2016 at 13:59

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2 Answers 2

Sorted by: Reset to default 10

The problem is that your ++ operator implicitly involves an assignment, and you can't assign a new value to a string constant. Note that

++2;

is also erroneous for the same reason.

In my understanding, ++ is similar to += 1.

So it will work for ++x as it will be evaluated to x+=1 or x=x+1, but ++'1' is a string literal and does not has left hand side value to assign, hence it fails

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