Why is number + string a string in javascript?

Question

Sort of trying out some quirks in javascript:

First I did

console.log("5" + 1);

This prints 51, this is normal right, both number and string have a + operator, but since string is the first variable it will convert 1 to a string.

Now when I did this:

console.log(1 + "5")

I expected output to be 6, as I thought it would convert string to a number. However, the magic output was 15.

Could anyone more experienced in javascript brighten this up for me?

Answer 1

Quoting ECMAScript spec The Addition operator ( + ) section:

7. If Type(lprim) is String or Type(rprim) is String, then
   Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

So the order doesn't matter here.

Answer 2

console.log(1 + "5") I expected output to be 6, as I thought it would convert string to a number. ...

But then what would you expect if you had written the following?

console.log(1 + " fine day")

or

console.log(1 + " answer(s) to my question")

There can be no assurance as a general rule that a string is convertible to a number. But any number can be converted to a string. That's why the conversion rules are written to move toward a type that is compatible. (In contexts where you as a programmer know that a string can be safely converted to a number, then you can do so explicitly so that the + operation is between two numbers. But that is not true in general for strings.)

In other contexts, this is also why small ints and low precision floats would be converted to large ints or double precision floats when operating on mixed types that the latter types. You can safely convert the limited forms into the larger forms, but you cannot safely go in the other direction in general. A small int can be represented as a big int or a double, but the other direction is not generally a safe conversion.

So conversion rules for operation on mixed types are written as much as is feasible to move toward mutually compatible types that are safe common types. It is left to the programmer to write explicit conversions for the special cases where a more general type can be safely converted to a more limited type.