Pfthb

In JavaScript, strings are immutable. That means any operation on them returns a new object. Methods like trim, replace and slice don't modify the existing string.

However, I was playing around in jsconsole and found an exception

string = "string";
string + 37;
=> "string37"
string;
=> "string"

The original string hadn't changed here. Here's what happens when I apply the increment operator on the string. Here I am expecting to see string returned.

string++;
=> NaN
string
=> NaN

I was trying to see whether this would return strinh, like it would in some other languages.

Regardless of whether string++ would work, it shouldn't modify existing string objects. Yet it did just that. Does anyone know why?

Here is a simple example:

var baz = "string";
baz = 5;

Certainly we have not modified the value of the string "string" by assigning baz to 5. Instead, we have simply done away with the string altogether and put the value 5 in its place. Similarly, your example does not alter any strings by assigning your variable to NaN.

You expect the variable string to be a constant, but it's not. Furthermore, constant variables and immutable values are different things. You haven't changed the immutable string "string" into NaN; you have substituted the immutable value "string" for a completely different value, NaN.

Your operations create new values by using the immutable string as an operand.

Consider a real constant variable in an environment that supports the const keyword:

> const foo = 5;
> foo++
 5

Now consider a constant variable that is an object:

> const bar = ;
> bar.baz = 5
> bar
 baz: 5 
> bar = 10;
> bar
 baz: 5 

In this case, we mutated the value (because the value is not immutable) but could not alter which value the variable contains. Your example does not use constant variables, so the variable string can freely change its value.

That ++ operator works similarly to:

string = string + 1;

So, you are re-assigning to that variable. (while the original string remains immutable)

If you run string = string + 1 yourself, you'll notice it results in string1. This is because the 1 is being converted into a string and then treated as an "append". (again, never modifying the original string value)

However, the ++ does not attempt to coerce anything, it simply attempts to add a non-number (ie: string) to a number (1) and the result is NaN.