How to return the response from an asynchronous call

I have a function foo which makes an asynchronous request. How can I return the response/result from foo?

I am trying to return the value from the callback, as well as assigning the result to a local variable inside the function and returning that one, but none of those ways actually return the response (they all return undefined or whatever the initial value of the variable result is).

Example of an asynchronous function that accepts a callback (using jQuery’s ajax function)

function foo() {
    var result;

    $.ajax({
        url: '...',
        success: function(response) {
            result = response;
            // return response; // <- I tried that one as well
        }
    });

    return result; // It always returns `undefined`
}

Example using Node.js:

function foo() {
    var result;

    fs.readFile("path/to/file", function(err, data) {
        result = data;
        // return data; // <- I tried that one as well
    });

    return result; // It always returns `undefined`
}

Example using the then block of a promise:

function foo() {
    var result;

    fetch(url).then(function(response) {
        result = response;
        // return response; // <- I tried that one as well
    });

    return result; // It always returns `undefined`
}