Using a block's return value in JavaScript

Question

On a lot of browsers I've tested, JavaScript blocks actually return a value. You can test it out in any console:

for(var i = 0; i < 10; i++) {
    var sqrt = Math.sqrt(i);
    if(Math.floor(sqrt) === sqrt) {
        i;
    }
}

The "return" value is the last square number, that is, 9! But since it isn't an expression I suppose, you can't do this:

for(var i = 0; i < 10; i++) {
    ...
} + 5

That doesn't work. It gives + 5, or 5, of course, because it's a separate statement. Putting the loop in parentheses obviously fails, and if a block is in parentheses (e.g. ({f(); r}) - doesn't work) it's treated as an object and throws a syntax error.

One way to take advantage of the return value, as such, is to use eval:

eval('for(var i = 0; i < 10; i++) {var sqrt = Math.sqrt(i);if(Math.floor(sqrt) === sqrt) {i;}}') + 5; // 14

But I'm obviously not going to want to use that if eval is the only solution. Is there a way to use a block's resultant value without using eval that I'm missing? I really like this feature :)

Answer 1

In JavaScript, statements return values of the Completion type (which is not a language type, but a specification type).

The Completion type is used to explain the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control. Values of the Completion type are triples of the form (type, value, target), where type is one of normal, break, continue, return, or throw, value is any ECMAScript language value or empty, and target is any ECMAScript identifier or empty.

Source: http://es5.github.com/x8.html#x8.9

So, eval() evaluates the program that has been passed in as source text. That program (like any JavaScript program) returns a Completion value. The second item in this Completion value (the "value" item) is returned by the eval() invocation.

So, with eval you are able to retrieve the completion value of an JavaScript program. I am not aware of any other method to accomplish this...

Answer 2

There is a proposal for ES7 to introduce a do expression which allows any block to be turned into an expression. A do expression evaluates a block and returns its completion value.

Using this syntax, which you can try out today with Babel using the syntax-do-expression and transform-do-expression plugins, your example would look like this:

function lastSquareNumber(val) {
    return do { for(var i = 0; i < val; i++) {
        var sqrt = Math.sqrt(i);
        if(Math.floor(sqrt) === sqrt) {
            i;
        }
    }}
}

console.log(lastSquareNumber(10));