Javascript String Assignment Operators

Question

How come I can use += on a string, but I cannot use -= on it?

For example...

var test = "Test";
var arr = "⇔"

test += arr;
alert(test);  // Shows "Test⇔"

test -= arr;
alert(test);  // Shows "NaN"

Answer 1

Because the + operator concatenates strings, but the - operator only subtracts numbers from each other.

As to the why -- probably because it is difficult to determine what people want to do when they subtract strings from each other.

For example:

"My string is a very string-y string" - "string"

What should this do?

Answer 2

The short answer is - it isn't defined to work with strings.

Longer answer: if you try the subtraction operator on two strings, it will first cast them to numbers and then perform the arithmetic.

"10" - "2" = 8

If you try something that is non-numeric, you get a NaN related error:

"AA" - "A" = NaN