javascript: determine functions return type

No, you are going to have to run the function and check the type of the resulting value.

This isn't Haskell - Javascript functions can return anything.

    function dosomething()
    {
        return true;
    }
    var myfunc=dosomething();
    if(typeof myfunc=="boolean") alert('It is '+typeof myfunc);

You can use

if(typeof myfunc=="boolean") or if(typeof myfunc=="number") or if(typeof myfunc=="string") 

or

if(typeof myfunc=="object") or if(typeof myfunc=="undefined") to determine the type.

Check what the type is:

    var x = typeof doSomething2('a');
    
    if (x == "string")
        alert("string")
    else if (x == "number")
        alert("number");
    else if (x == "undefined")
        alert('nothing returned');
    else if (x == "boolean")
        alert("boolean");
    else
        alert(x);

A simple shorthand would of course be, assuming "undefined" would be fine to return instead of "nothing returned":

alert(typeof doSomething2('a'))

Example use:

[undefined, 'stringyMeThingy', 42, true, null].forEach(x => console.log(typeof x))