Question

I just spend long time digging through callbacks in promises wondering why some callbacks are not called. In the end problem was with incorrect declaration, instead of

       Promise.when(null).then(function () {
            var p = new Promise();
            p.fail(new Error("some message"));
            return p;
        }).then(function () {
            console.log("success");
        }, function (err) {
            console.log("failure");
        });

I did

       Promise.when(null).then(function () {
            var p = new Promise();
            p.fail(new Error("some message"));
            return p;
        }).then(function () {
            console.log("success");
        }), function (err) {
            console.log("failure");
        };

Regardless of Promise implementation details it boils down to one thing:

function(){};//throws SyntaxError
"something valid, or function call", function(){};//no error

I would like someone to explain this to me. Why first is throwing SyntaxError while latter is valid (at least in browser console)? It seems to declare anonymous function.
Same thing happens when I try

eval("function(){};//throws SyntaxError")
eval("'lala',function(){};//no error")

So can someone explain me why first is not valid while latter is ?

Answer 1

A statement that begins with the keyword “function” must be a valid function declaration statement. That requires a name for the function.

In an expression (or expression statement), that rule is different; no name is necessary because the function acts as a value in that context. No name is required then.

Thus this:

function myFunction() {
  // ...
}

is a function declaration statement. This:

5 + function() {};

is an expression statement (a silly one, but JavaScript is ok with that). Similarly, your example that results in no error is an expression with the comma operator:

"lala", function() {};

The keyword “function” does not appear at the beginning of either of those two expressions, so the parser does not insist on a name for the function.