Question

{1 + ''} + 10 // 10
{1 + ''} + '' // 0

Why does this happen? Do BlockStatements return 0, and why?

Answer 1

Do BlockStatements return 0…?

No, blocks return the value of the last expression within them. You can see this by just doing:

{1 + 8}

…in the JavaScript console, which will show 9.

{1 + ''} + 10 // 10

{1 + ''} + '' // 0

Why does this happen?

Because although the block does return a value, that value is not used. {1 + ''} + 10 // 10 code is evaluated as two distinct items:

{1 + ''} // "1"
+10      // 10

…or writing those with standard indentation and semicolons:

{
    1 + '';
}
+10;

…and you’re seeing the result of the second one, as though the first one weren’t there at all. The + there isn’t the addition operator, it’s the unary + (similar to the unary -, but it doesn’t change the sign of its operand). +10 is, of course, 10; and +'' is 0 because applying the operator to a string converts the string to a number, and Number('') is 0.

You can prove that you’re seeing the unary + rather than the addition operator by trying this:

{1 + ''} * 10

…which is really

{
    1 + '';
}
*10;

It fails with a syntax error because there is no unary *.

As Felix kindly points out in the comments below, for the + in your example to be the addition operator (which would have ended up concatenating strings, in your case), it would have to be between two expressions, and a block is a statement, not an expression.