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

Generally, programming languages don't define subtraction for strings. += isn't really addition in the first place, it's concatenation.

Answer 2

That's because the minus sign is not a valid String operator, whereas the plus sign is overloaded to handle both Numbers (addition operator) and Strings (concatenation operator).

What results were you hoping to get from this?

Answer 3

Because the +(plus sign) is also the string concatenation operator, while the -(minus sign) only applies to subtraction. If JavaScript can append 2 strings together it won't complain, but if you try to subtract 2 strings, it just doesn't make any sense.

Answer 4

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 5

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

Answer 6

As all said, the -= operator is not overloaded to work with Strings, it only works with Numbers.

If you try to use it with strings, the operator will try to convert both operands to Number, that is why you are getting NaN, because:

isNaN(+"foo"); // true

To get rid of the arr content on your test string, you can replace it:

var test = "Test",
    arr = "⇔"

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

test = test.replace(arr, ""); // replace the content of 'arr' with "" on 'test'
alert(test);  // Shows "Test"