Question

Is there a way in javascript to determine the return type(if any) of a function?

example:

function doSomething(){
   return true;
}

to returned type is boolean.

example 2:

function doSomething2(x){
    if(x=="a") return 1;//number
    else return "bad x"; //string
}

Answer 1

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))