JavaScript increment unary operator (++) on strings

Question

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:

Prefix Increment Operator evaluation steps

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

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
– Bergi, Commented Oct 12, 2016 at 13:59

Pointy · Accepted Answer · 2016-10-12 13:58:05Z

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.

answered Oct 12, 2016 at 13:58

Pointy

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2 Comments

deceze Over a year ago

…which presumably happens in 5. Call PutValue(expr, newValue)?

iulian Over a year ago

Oh.. I missed that out. Thanks for the answer!

Rajesh Dixit · Accepted Answer · 2016-10-12 13:59:33Z

4

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

answered Oct 12, 2016 at 13:59

Rajesh Dixit

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3 Comments

Bergi Over a year ago

You can even literally do '1' = Number('1')+1 and will get the same error

Rajesh Dixit Over a year ago

Because left hand side value is not valid. you cannot have '1' as variable name

iulian Over a year ago

Oh.. I missed that out. Thanks for the answer!

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